LESSONS  AND  PRACTICAL  NOTES 


ON 


STEAM, 

THE  STEAM  ENGINE,  PROPELLERS, 

ETC.,  ETC., 

FOR 

YOUNG  MARINE  ENGINEERS, 

STUDENTS,  AND  OTHERS. 


BY  THE  LATE 


W.  H.  KING,  U.  S.  N. 


REVISED  BY 

CHIEF  ENGINEER  J.  W.  KING,  II.  S.  N. 

[SECOND  EDITION,  ENLARGED.] 
NEW    YORK: 

D.    VAN    NOSTRAND    192    BROADWAY, 

LONDON: 

TRUBNEK    &    COMPANY. 
1862. 


Entered,  according  to  Act  of  Congress,  in  the  year  1860, 

BY  J.  W.  KING, 

In  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for  the  Southern  District  of 

New  York. 


JOH\  F.  TROW, 

PH'VrKH,  FTIRKOTYPER,   AND   ELICTKOTf PIE, 

50  Greene  Street,  New  York. 


PREFACE  TO  SECOND  EDITION. 


THE  flattering  reception  of  the  first  edition  or  my 
lamented  brother's  work  has  encouraged  me  to  cause 
the  issue  of  a  second,  with  a  few  additions  on  the 
elements  of  machinery,  withheld  from  the  first  edi- 
tion. As  the  elements  of  machinery,  like  physical 
laws,  must  be  thoroughly  understood  by  the  young 
engineer,  before  eminence  in  his  profession  can  be 
securely  attained,  and  as  but  few  young  men  learning 
engineering  practically,  cultivate  this  knowledge  un- 
derstandingly,  if  at  all,  it  has  been  considered  proper 
to  devote  a  short  space  to  the  subject,  giving  ex- 
amples and  explanations,  both  thorough  and  plain. 

J.  W.  KING, 

*  Chief  Engineer,  U.  S.  Navy. 


CONTENTS. 


INTRODUCTION,  PAGE  5. 
CHAPTER  I. 


Steam,  7.  Mechanical  Effect,  9.  Expansion  of  Steam,  12.  Table  of  Hyperbolic 
Logarithms,  14.  Back  Pressure,  16.  Gain  by  Expanded  Steam,  18. 

EXPANSION    VALVES. 

Sickel's,  19.    Stevens',  22.     Allen  &  Wells',  23. 

SLIDE   CUT-OFFS. 

Explanation,  24.     Gridiron  Valve,  26.     Wabash  Valve,  29. 

OTHER   KINDS    OF   VALVES. 

Double  Poppet,  30.  Single  Poppet,  81.  Hornblower's,  32.  Box  Valve,  33- 
Equilibrium  Slide,  34.  Double  Slide  Valve,  34.  Piston  Valve,  35.  Long 
D  Slide,  36.  Short  D_  Slide,  37.  Worthington  Pump  Valve,  38.  Pitts- 
burg  Cam,  39. 

CHAPTER  II. 

THE    INDICATOR   AND    INDICATOR   DIAGRAMS. 

The  Indicator,  41.  Cylinder  Diagrams,  44.  Air-pump  Diagrams,  56.  Power 
Required  to  Work  the  Air-pump,  60. 

CHAPTER  III. 

THE    HYDROMETER. 

The  Hydrometer,  62.  Loss  by  Blowing-off,  64.  Gain  by  the  Use  of  Heaters, 
68.  Injection  Water,  71.  Evaporation,  72.  Steam  and  Vacuum  Gauges,  75. 


4  CONTENTS. 

CHAPTER  IV. 

CAUSALTIES,    ETC. 

Broken  Eccentric,  79.  Leaking  Vessel,  79.  Irregular  Feed,  80.  Foaming,  81. 
Ilot  Condenser,  83.  Getting  Under  Way,  85.  Coming  into  Port,  86. 
Scaling  Boilers,  88.  On  Coming  to  Anchor,  etc.,  89.  Management  of 
Fires,  90.  Patching  Boilers,  93.  Sweeping  Flues,  95.  Ash  Pits,  95. 
Smoke-pipe  Stays,  96.  Grate  Bars,  &c.,  96.  Broken  Air-pump,  97.  Bro- 
ken Cylinder-head,  98.  Selection  of  Coal,  98.  Safety  Valve,  99. 

CHAPTER  V. 

MISCELLANEOUS. 

Theory  of  the  Paddle  Wheel,  101.  Centre  of  Pressure,  114.  Screw  Propeller, 
116.  Altering  the  Pitch,  132.  Parallel  Motion,  133.  Strength  of  Mate- 
rials, 136.  Surface  Condensers,  141.  Cylindrical  Boilers,  145.  Boiler 
Explosions,  148.  Horse  Power,  150.  Vibration  of  Beams,  152.  Marine 
Economy,  154.  Limit  to  Expansion,  155.  Gravity,  156.  Displacement  of 
Fluids,  158.  Temperature  of  Condensers,  159. 

APPENDIX. 

MATERIALS. 

How  to  Test  Iron,  162.  Cast  Iron  and 'Steel,  163.  Tenacity  of  Materials,  164. 
Resistance  to  Torsion,  165.  Results  of  Repeated  Heating  Bar  Iron,  166. 
Strength  of  Joints  of  Boiler  Plates,  167. 

THE    ELEMENTS   OF   MACHINERY. 

Motion,  169.  Application  of  Power,  1 70.  The  Lever,  172.  Inclined  Plane,  175. 
Wheel  and  Axle,  177.  The  Pulley,  177.  The  Screw,  181.  The  Wedge,  182. 
Table  of  Pressure,  Temperature,  and  Volume  of  Steam,  183. 


INTRODUCTION. 

WETTING  a  book  and  then  apologizing  for  having 
written  it,  is  hardly  in  accordance  with  our  convic- 
tions ;  but  considering,  nevertheless,  the  eminent  tal- 
ent which  has  preceded  us  upon  the  subject  we  have 
taken  up,  a  few  remarks  of  explanation  may  not  be 
out  of  place.  Books  heretofore  appearing  on  the 
steam  engine,  have  been  of  two  classes,  or  the  work 
itself  has  been  divided  into  two  parts — the  one  for  the 
theorist,  the  other  for  the  practical  man.  In  the  one 
case  long  mathematical  formulas  have  bsen  produced, 
and  in  the  other  nothing  but  simple  rules.  The  prac- 
tical man,  therefore,  who  has  not  had  the  advantage 
of  a  mathematical  education,  has  nothing  presented  to 
him  but  the  bare  rules,  which  he  is  compelled  wholly 
to  reject,  or  take  entirely  upon  trust.  Besides,  these 
works  extend  over  numerous  volumes,  the  study  of 
which  involve  much  time,  labor,  and  expense,  and 
which  usually  disheartens  the  practical  man  before  he 
has  made  much  progress.  Having  had  many  of  these 
difficulties  to  surmount  in  our  earlier  studies  of  the 
steam  engine,  we  were  led  to  the  course  of  keeping  a 
Steam  Journal,  in  which  we  noted,  from  time  to  time, 
as  we  progressed,  whatever  we  thought  important,  and 
was  made  clear  to  our  mind  ;  and  this  course  we  would 
also  recommend  the  young  student ;  for,  however  well 


6  INTKODUCTION. 

it  may  be  to  study  books  containing  other  mens1 
thoughts,  when  we  write  we  are  led  to  the  habit  of 
thinking  for  ourselves,  which  is  of  the  highest  impor- 
tance ;  and,  by  keeping  a  journal,  we  have  also  the 
very  great  advantage  of  having  always  at  our  com- 
mand, in  a  condensed  form,  those  things  which  are  the 
more  important,  and  which  can  be  referred  to  at  any 
time. 

Much  of  the  present  work  has  been  taken  from  the 
Author's  Journal,  and  the  remainder  has  been  sup- 
plied, from  time  to  time,  as  he  found  leisure  from  his 
hours  of  business. 

Our  object  has  not  been  so  much  to  supply  want- 
ing information,  as  to  direct  the  student  into  the  habit 
of  thinking  and  reasoning  for  himself  on  those  subjects 
which  may  be  presented  for  his  consideration,  and 
which,  in  order  that  he  may  become  eminent  in  his 
profession,  he  must  thoroughly  understand.  It  is  not 
sufficient  to  assert  that  Newton  said  this,  or  somebody 
else  said  that.  The  reasons  why  they  said  it,  and  the 
fundamental  principles  upon  which  they  based  their 
conclusions,  are  necessary  to  be  understood,  in  order 
to  have  a  clear  understanding  of  the  subject;  and  if 
we  have  succeeded  in  making  any  thing  more  clear, 
or  in  rendering  any  service  to  that  class  of  persons 
who  are  eagerly  seeking  for  information,  but  who  re- 
quire some  assistance  to  direct  them  in  the  proper 
channel,  our  only  object  in  launching  this,  our  little 
bark,  on  the  troubled  sea  of  authorship,  is  fully  accom- 
plished, conscious  all  the  while,  however,  of  the  many 
imperfections  it  contains. 


LESSONS  A^D  PRACTICAL  NOTES. 


CHAPTER  I. 


STEAM. 


STEAM  is  a  thin,  elastic,  invisible  fluid,  generated 
by  the  application  of  heat  to  any  liquid,  usually  water. 
That,  however,  which  is  generated  while  the  water 
is  in  a  state  of  ebullition,  is  alone  generally  termed 
steain,  while  that  which  is  formed  while  the  surface  of 
the  water  is  quiescent,  is  denominated  vapor — a  dis- 
tinction, to  our  mind,  without  much  difference. 

The  mean  pressure  of  the  atmosphere  at  the  sur- 
face of  the  ocean  is  equal  to  14.Y  pounds  per  square 
inch,  or  is  equivalent  in  pressure  to  a  column  of  mer- 
cury 29.9212  inches  in  height.  Under  this  pressure, 
fresh  water  boils  at  a  temperature  of  212°  Fahrenheit. 

The  212°  is,  however,  not  the  total  number  of  de- 
grees in  the  steam,  but  simply  that  which  is  indicated 
by  the  thermometer,  and  which  is  termed  sensible 
heat ;  for  we  all  know  that  to  raise  water  from  the 
freezing  to  the  boiling  point  requires  a  certain  time, 
and  a  certain  amount  of  fuel ;  and  we  know  further, 
that  when  the  water  commences  to  boil,  it  does  not  all 
evaporate  at  once,  but  that  the  evaporation  goes  on 


8  STEAM. 

gradually,  and  the  time,  and  hence  the  fuel  required 
to  evaporate  it,  is  much  greater  than  that  required  to 
raise  it  from  the  freezing  to  the  boiling  point.  This 
extra  heat  must  have  gone  off  somewhere,  and  must 
be  in  the  steam,  but  as  it  is  not  indicated  by  the  ther- 
mometer, it  is  termed  latent  heat.  When  the  steam  is 
reconverted  into  water,  the  latent  heat  becomes  again 
sensible,  which  is  evidenced  by  the  large  amount  of 
water  required  to  condense  a  small  amount  in  the 
shape  of  steam.  The  precise  ratio  the  one  bears  to 
the  other  shows  the  latent,  compared  with  the  sensible 
heat. 

The  subject  of  latent  heat  has  been  one  of  unusual 
interest,  ever  since  the  invention  of  the  steam  engine, 
and  numerous  theories  have  been  advanced,  and  nu- 
merous experiments  made — some  of  them  not  very 
carefully — in  order  to  determine  the  exact  law  it  fol- 
lowed ;  but  none,  up  to  Regnault's  time,  seem  to  have 
settled  the  subject  satisfactorily.  Some  maintained 
that  the  latent  heat  of  steam  was  a  constant  quantity, 
some  that  the  sum  of  sensible  and  latent  heat  was  a 
constant  quantity,  and  that  quantity  was  1202°  Fahr- 
enheit. This  was  the  most  popular  theory,  and  was 
the  one  generally  adopted  by  engineers.  Others, 
again,  maintained  that  neither  the  sensible,  latent,  nor 
sum  of  the  sensible  and  latent  heats,  were  a  constant 
quantity,  but  that  they  all  varied.  The  exact  ratio, 
however,  in  which  they  varied  was  not  established 
until  Regnault  undertook  his  able  series  of  experi- 
ments at  the  instigation  of  the  French  Government. 
These  are  the  latest  and  most  reliable  experiments, 
and  we  subjoin,  therefore,  a  table  compiled  from  his 
labors,  which  we  earnestly  recommend  to  the  attention 
of  the  reader. 


MECHANICAL   EFFECT. 


9 


REGNAULT'S  EXPERIMENTS. 


Degrees  of  heat  contained  in  saturated  steam,  in  Fahrenheit  degrees 
of  heat  and  English  inches. 


Iff! 

Corresponding  elastic 

if.  3 

f|f| 

Corresponding  elastic 

|loSj 

force 

Ji|| 

ijlj 

force 

~JJ| 
1-S^ 

0)  3^,0 

|J  if 

In 

In  Atmo- 

!|fej 

In 

In  Atmo- 

I^JI 

1*6* 

Inches. 

spheres. 

ir°° 

Ill's 

Inches. 

spheres. 

ll    b 

°Fah. 

32 

0.1811 

0.006 

1123.70 

248 

517116 

1.962 

1189.58 

50 

0.3606 

0.012 

1129.10 

266 

79.9321 

2.671 

1194.98 

68 

0.6846 

0.023 

1134.68 

284 

106.9930 

3.576 

1200.56 

86 

1.2421 

0.042 

1140.16 

302 

140.9930 

4.712 

1205.96 

104 

2.1618 

0.072 

1145.66 

320 

183.1342 

6.120 

1211.54 

122 

3.6212 

0.121 

1151.06 

338 

234.7105 

7.844 

1216.94 

140 

5.8578 

0.196 

1156.64 

356 

297.1013 

9.929 

1222.52 

158 

9.1767 

0.306 

1162.04 

374 

371.7590 

12.425 

1227.92 

176 

13.9621 

0.466 

1167.62 

392 

460.1943 

15.380 

1233.50 

194 

20.6869 

0.691 

1173.02 

410 

560.9673 

18.848 

1238.90 

212 

29.9212 

1.000 

1178.60 

428 

684.6584 

22.882 

1244.48 

230 

42.3374 

1.415 

1184.00 

446 

823.8723 

27.535 

1249.88 

FIG.  1. 


FIG.  2. 


MECHANICAL    EFFECT. 

We  will  now  take  into  consideration  the  mechani- 
cal effect  of  steam,  and  a  common-place  demonstration 
will  serve  our  purpose. 

Suppose  a  cylinder,  A,  Fig. 
1,  to  be  one  square  inch  in 
area  of  cross  section,  and  fitted 
with  a  steam  tight  piston,  at- 
tached by  means  of  a  flexible 
cord  to  the  weight  £,  which  is 
of  sufficient  size  to  balance  the 
weight  of  the  piston,  and  all 
the  parts  to  work  without 
friction.  Now  suppose  a  quan- 
tity of  water,  equal  to  one 


O 


A 


0 


cubic  inch,  to  be  placed  in  the  bottom  of  this  cylinder, 


10  MECHANICAL    EFFECT. 

and  a  fire  to  be  lighted  under  it.  The  temperature 
of  the  water  will  gradually  rise  until  it  attains  212°, 
when  it  will  commence  to  boil,  and  the  piston  will 
soon  begin,  and  continue  to  rise — if  the  cylinder  be 
long  enough — until  it  obtains  a  height  of  1700  inches 
from  the  base.  This  IT 00  is  the  volume  of  steam  at 
atmospheric  pressure,  the  water  being  1,  from  which  it 
is  generated.  If,  now,  we  suppose  to  be  added  to  the 
weight,  3,  another  weight  equal  to  the  pressure  of  the 
atmosphere — or  a  fraction  less,  so  that  motion  may  en- 
sue— and  the  steam  under  the  piston  to  be  condensed, 
the  piston  will  return  to  the  bottom  of  the  cylinder 
by  the  pressure  of  the  atmosphere,  through  a  space 
of  1700  inches,  and  will  have  raised  the  extra  weight  of 
14.7  Ibs.  appended  to  0,  up  that  distance.  Hence  this 
cubic  inch  of  water,  by  its  evaporation,  produced  a 
mechanical  effect  of  raising  14.7  pounds  through  a 
space  of  1700  inches  =  (14.7  X  1700).=  24,990  pounds 
through  one  inch. 

Let  us  now  take  another  cylinder,  B,  Fig.  2,  similar 
in  every  respect  to  A,  excepting  that  the  piston  has  a 
weight  laid  upon  it  equal  to  the  pressure  of  the  atmo- 
sphere, viz.,  14.7  pounds,  and  suppose  a  fire  to  be 
lighted  under  this  cylinder.  The  water,  as  in  the 
other  case,  will  be  heated  up  to  the  boiling  point,— 
which,  in  this  case,  will  be  250°,  corresponding  to  the 
pressure  of  two  atmospheres — when  it  will  commence 
to  evaporate,  and  the  piston  will  rise  until  it  obtains  a 
height  of  900  inches  from  the  base,  this  being  the 
volume  of  steam  under  the  pressure  of  two  atmo- 
spheres, water  being  1.  If,  now,  we  suppose  this  pis- 
ton to  be  fixed  where  it  is,  the  weight  removed  from 
the  top  of  it  and  applied  to  <?,  then  the  steam  condensed 
and  the  piston  unfixed,  it  will  return  to  the  bottom  of 


MECHANICAL   EFFECT.  11 

the  cylinder,  raising  the  weight  applied  to  c,  up  a  dis- 
tance of  900  inches.  Now,  then,  since  the  weight  of 
14.7  Ibs.  was  first  raised  900  inches  on  the  top  of  the 
piston,  and  afterwards  raised  the  same  distance  by  be- 
ing attached  to  c,  the  total  distance  moved  =  (900  X  2) 
=  1800  inches,  which  is  equal  to  (14.7  X  1800)  =  26460 
pounds  raised  one  inch.  The  difference,  therefore,  be- 
tween the  work  done  in  the  first  and  second  case 
=  (26460— 24990)  =  1470  Ibs.  raised  one  inch  high, 
which  is  5.88  per  cent,  of  the  first  number.  If  this 
extra  work  was  obtained  without  any  extra  fuel,  which 
would  be  the  case  were  the  total  heat  in  steam  at  all 
temperatures  a  constant  quantity,  it  would  be  all  gain, 
but  as  such  is  not  the  case,  and  as  more  heat  is  required 
in  the  latter  than  in  the  former  case,  we  will  see  what 
this  amounts  to,  and  the  difference  between  this  loss 
and  the  other  gain  will  show  the  true  gain.  In  the 
first  instance,  it  will  be  seen  that  the  total  heat  in  the 
steam  was  1178.6°,  and  in  the  second,  1190° ;  hence, 
supposing  the  water  in  both  cases  to  be  at  a  tempera- 
ture of  100°  before  the  fires  are  lighted — which  is 
about  the  temperature  at  which  water  is  fed  into  ma- 
rine boilers — there  would  be  required  in  the  latter 
case  (1190°-  100)  =  1090°  from  the  fuel,  and  in  the 
former  case  (1178.6°- 100°)  =  1078.6°  from  the  fuel ; 
difference  11.4°,  which  is  1.057  per  cent,  of  1078.6°. 

The  extra  fuel,  therefore,  required  under  the  pres- 
sure of  two  atmospheres  is  1.057  per  cent.,  and  the 
extra  work  done  is  5.88  per  cent.,  leaving  a  gain  of 
4.823  per  cent. 

In  the  same  way  we  could  ascertain  what  the  gain 
would  be  at  any  other  pressure,  either  higher  or  lower ; 
but  these  examples  suffice  to  show  that  the'  higher  the 
pressure  of  the  steam,  the  greater  is  the  mechanical 


12  EXPANSION   OF   STEAM. 

effect  with  the  same  amount  of  fuel,  but  the  gain  is 
small,  and  in  practice,  therefore,  where  great  accuracy 
is  not  required,  it  is  neglected  altogether. 

Starting,  therefore,  from  the  assumption  that  the 
mechanical  effect  performed  by  the  same  amount  of 
fuel  is  the  same,  no  matter  what  the  pressure  may  be 
under  which  the  steam  is  generated,  we  shall  proceed 
to  the  study  of  the 

EXPANSION    OF   STEAM. 

Opening  a  communication  with  the  cylinder  and 
shutting  it  off  again  before  the  piston  arrives  at  the 
end  of  the  stroke  is  called  expansion  of  steam,  or  work- 
ing steam  expansively.  Thus,  supposing  steam  to  be 
admitted  into  the  cylinder  until  the  piston  arrives  at 
half  stroke,  and  the  communication  then  to  be  shut  off, 
the  steam  already  in  the  cylinder,  by  its  expansion, 
will  force  the  piston  to  the  end  of  the  stroke ;  by 
which  arrangement  we  gain  all  the  work  performed 
after  the  steam  is  cut-off. 

FIG.  s.  Take,  for  instance,  a  cylinder,  A,  Fig.  3, 

two  units  in  length,  one  unit  in  area  of 
cross  section,  and  an  initial  pressure  of  1, 
the  work  performed  during  the  first  half 
stroke,  i.  e.,  while  the  piston  travels  from 
b  to  £,  will  be  1  X  1  X  1  (area  x  pressure 
X  distance  travelled  =)  1,  and  the  work 
.69  performed  during  the  latter  half  stroke  — 
1  X  .69  X  1  =  -69,  the  total  work,  there- 

fore,  performed  throughout    the   stroke 

=  1.69.  Now,  if  the  steam,  instead  of  be- 
ing expanded  from  c  to  d,  had  been  exhausted  at  <?,  the 
total  work  performed  would  have  been  only  1  instead 


EXPANSION    OF   STEAM. 


13 


of  1.69,  and  the  quantity  of  steam  would  have  been  the 
same,  hence  we  see  that  by  cutting  off  at  one  half  the 
same  steam  performs  69  per  cent,  more  work.  This  69 
per  cent,  is  what  is  termed  the  gain  in  cutting  off,  but  it 
does  not,  however,  represent  the  saving  in  fuel,  as  we 
will  show  presently  ;  but  before  proceeding  to  illus- 
trate that  subject  we  will  explain  to  the  student,  from 
what  source  we  derive  this  69. 

Marriotte's  law  of  gases  is,  that  the  spaces  occupied 
are  inversely  as  the  pressures.  That  is  to  say,  if  steam 
of  20  pounds  pressure  per  square  inch,  be  allowed  to 
expand  into  double  the  space,  the  pressure  will  be 
10  Ibs.  ;  if  triple,  6|  Ibs.  ;  if  four  times,  5  Ibs.  ;  if  five 
times,  4  Ibs.,  and  so  on.  This  theory  would  be  liter- 
ally correct  did  the  temperatures  remain  constant  ;  but 
as  the  temperature  of  all  gases  becomes  reduced  by  ex- 
pansion, the  law  does  not  hold  good  ;  nevertheless,  in 
the  steam  engine,  where  there  are  so  many  extraneous 
circumstances  which  practically  affect  all  calculations 
appertaining  to  the  same,  it  is  considered  all  that  is 
ever  required,  and  from  its  extreme  simplicity  is  uni- 
versally adopted. 

From  this  law  the  pressure  can  be 
ascertained  approximately  by  dividing 
the  cylinder  into  a  number  of  equal 
parts,  say  eight,  ascertaining  the  pres- 
sure at  each  of  those  points,  and  taking 
the  mean.  If  the  initial  pressure,  as 
before,  be  supposed  to  be  unity,  the 
pressure  at  each  of  the  first  four  divi- 
sions cutting  off  at  half  stroke  will  be 
1  ;  at  the  fifth  division  (|  =)  .8  ;  at  the 
6th  (|)  =  .6666;  at  the  7th  (4-  =)  .5714; 
at  the  8th  (£  =)  .5  ;  the  mean  pressure,  therefore,  by 


Fio.  4. 


.8000 


.5714 
.5000 


14 


TABLE    OF   HYPEKBOLIC    LOGAKITHMS. 


this  process,  after  the  steam  is  cut  off  —  .6345,  and  the 
mean  pressure  before  it  is  cut  off  =  1,  the  mean,  there- 
fore, throughout  the  stroke  =  (-  f^--  -)  =  .8172.—- 

This,  however,  is  only  an  approximation,  and  in  order- 
to  arrive  at  any  degree  of  accuracy,  the  divisions  would 
have  to  be  very  numerous,  which  would  render  the 
operation  tedious  and  lengthy.  Fortunately,  however, 
we  can  dispense  with  this  part  of  the  calculation  alto- 
gether, for  the  Naperian  or  Hyperbolic  logarithms,  as 
set  forth  in  the  following  table,  furnish  to  our  hand 
the  ratios  of  pressures : 

TABLE  OF  HYPERBOLIC  LOGARITHMS. 


o 

.2 

.2 

_o 

2 

£ 

1* 

1 

,Q  si 

M 

J>  ^ 

1 

ja  so 

o 

"3   • 
,0  t* 

S 
£ 

Is 

§ 

3 

»J 

£ 

>> 

| 

|3 

1 

r 

a 

H 

a 

a 

H 

1.05 

.049 

3.05 

1.115 

5.05 

1.619 

7.05 

1.953 

9.05 

2.203 

1.1 

.095 

3.1 

1.131 

6.1 

1.629 

7.1 

1.960 

9.1 

2.208 

1.15 

.140 

3.15 

1.147 

5.15 

1.639 

7.15 

1.967 

9.15 

2.214 

1.2 

.182 

3.2 

1.163 

6.2 

1.649 

7.2 

1.974 

9.2 

2.219 

1.25 

.223 

3.25 

1.179 

5.25 

1.658 

7.25 

1.981 

9.25 

2.225 

1.3 

.262 

3.3 

1.194 

5.3 

1.668 

7.3 

1.988 

9.3 

2.230 

1.35 

.300 

3.35 

1.209 

5.35 

1.677 

7.35 

1.995 

9.35 

2.235 

1.4 

.336 

3.4 

1.224 

5.4 

1.686 

7.4 

2.001 

9.4 

2.241 

1.45 

.372 

3.45 

1.238 

5.45 

1.696 

7.45 

2.008 

9.45 

2.246 

1.5 

.405 

3.5 

1.253 

6.5 

1.705 

7.5 

2.015 

9.5 

2.251 

1.55 

.438 

3.55 

1.267 

6.55 

1.714 

7.55 

2.022 

9.55 

2.257 

1.6 

.470 

3.6 

1.281 

5.6 

1.723 

7.6 

2.028 

9.6 

2.262 

1.65 

.500 

3.65 

1.295 

6.65 

1.732 

7.65 

2.035 

9.65 

2.267 

1.7 

.531 

3.7 

1.308 

5.7 

1.740 

7.7 

2.041 

9.7 

2.272 

1.75 

.560 

3.75 

1.322 

6.75 

1.749 

7.75 

2.048 

9.75 

2.277 

1.8 

.588 

3.8 

1.335 

5.8 

1.758 

7.8 

2.054 

9.8 

2.282 

1.85 

.615 

3.85 

1.348 

6.85 

1.766 

7.85 

2.061 

9.85 

2.287 

1.9 

.642 

3.9 

1.361 

6.9 

1.775 

7.9 

2.067 

9.9 

2.293 

1.95 

.668 

3.95 

1.374 

5.95 

1.783 

7.95 

2.073 

9.95 

2298 

2. 

.693 

4. 

1.386 

6. 

1-792 

8. 

2.079 

10. 

2.303 

2.05 

.718 

4.05 

1.399 

6.05 

1.800 

8.05 

2.086 

15. 

2.708 

2.1 

.742 

4.1 

1.411 

6.1 

1.808 

8.1 

2.092 

20. 

2.996 

2.15 

.765 

4.15 

1.423 

6.15 

1.816 

8.15 

2.098 

25. 

3.219 

2.2 

.788 

4.2 

1.435 

6.2 

1.824 

8.2 

2.104 

30. 

3.401 

2.25 

.811 

4.25 

1.447 

6.25 

1.833 

8.25 

2.110 

35. 

3.555 

2.3 

.833 

4.3 

1.459 

6.3 

1.841 

8.3 

2.116 

40. 

3.689 

2.35 

.854 

4.35 

1.470 

6.35 

1.848 

8.35 

2.122 

45. 

3.807 

2.4 

.875 

4.4 

1.482 

6.4 

1.856 

8.4 

2.128 

50. 

3.912 

2.45 

.896 

4.45 

1.493 

6.45 

1.864 

8.45 

2.134 

55. 

4.007 

2.6 

.916 

4.5 

1.504 

6.5 

1.872 

8.5 

2.140 

60. 

4.094 

2.55 

.936 

4.55 

1.515 

6.55 

1.879 

8.55 

2.146 

65. 

4.174 

2.6 

.956 

4.6 

1.526 

6.6 

1.887 

8.6 

2.152 

70. 

4.248 

2.65 

.975 

4.65 

1.537 

6.65 

1.895 

8.65 

2.158 

75. 

4.317 

2.7 

.993 

4.7 

1.548 

6.7 

1.902 

8.7 

2.163 

80. 

4.382 

2.75 

1.012 

4.75 

1.558 

6.75 

1.910 

8.75 

2.169 

85. 

4.443 

2.8 

1.032 

4.8 

1.569 

6.8 

1.917 

8.8 

2.175    ' 

90. 

4.500 

2.85 

1.047 

4.85 

1.579 

6.85 

1.924 

8.85 

2.180 

95. 

4.554 

2.9 

1.065 

4.9 

1.589 

6.9 

1.931     i 

8  0 

2.186 

100. 

4.605 

2.95 

1.082 

4.95 

1.599 

6.95 

1.939     ! 

8.!)"> 

2.192 

1000. 

6.908 

3. 

1.099 

6. 

1.609    , 

7. 

1.94'i 

!) 

2.197 

10000. 

9.210 

EXPANSION    OF   STEAM.  15 

The  hyperbolic  logarithm  of  any  number  can  be 
found  by  multiplying  the  common  logarithm  by 
2,30258509. 

From  the  nature  of  hyperbolic  logarithms  they  are 
tli us  very  useful  in  working  steam  expansively. 

Let  the  Line  A,  B,  Fig.  5,  represent        Fl0-5- 
the  pressure  of  steam — which  we  will  as- 
sume to  be  unity — at  the  time  the  cut-off 
valve  closes ;  C,  D,  half  the  length  of  A, 
B,  and  the  line  A,  C,  a  hyperbolic  curve, 
.69+  from  the  table  gives  the  mean  length  A 
of  all  the  ordinates,  1,  2,  3,  4,  <fec.,  which 
before  we  had  to  arrive  at  by  approxima- 
tion.   If  the  cut-off  valve,  instead  of  closing 
at  half  stroke,  had  closed  at  some  other 
point,  say,  when  the  piston  had  traveled 
only  one-fourth  its  distance,  C,  D,  would  be  one-fourth 
of  0,  £,  and  the  curve  A,  C,  would  have  extended  from 
a  to  c,  giving  1.38+  as  a  mean  of  all  the  ordinates 
below  <z,  1). 

All  we  require  then  in  working  examples  in  ex- 
pansion of  steam,  according  to  Marriotte's  law,  is  to 
know  the  initial  pressure  and  point  of  cutting  off,  from 
which  we  can  deduce  the  mean  pressure,  pressure  at 
the  end  of  the  stroke,  percentage  of  gain,  <fec.,  by 
having  before  us  a  table  of  hyperbolic  logarithms ; 
but  if  it  be  required  to  make  such  calculations,  when  a 
table  of  this  kind  is  not  come-at-able,  it  can  be  done  in 
the  manner  we  have  previously  shown. 

EXAMPLE  1st.  Suppose  you  have  a  cylinder  in 
which  you  are  using  steam  of  20  pounds  pressure  per 
square  inch,  inclusive  of  the  atmosphere,  and  cut  off  at 
half  stroke,  what  is  the  mean  pressure,  pressure  at  the 
end  of  the  stroke,  and  per  centage  of  gain. 


16  BACK   PRESSURE. 

Ans.  \st.  From  the  foregoing  considerations  we 
know  that  had  the  pressure  of  steam  been  1  pound  in- 
stead of  20,  all  we  would  have  to  do  would  be  to  take 
.69314  out  of  the  table,  add  1  to  it  and  divide  by  2 ; 
therefore  to  find  the  mean  pressure  we  have  this  rule : 
As  the  number  of  times  the  steam  is  expanded,  is  to  tlie 
hyperbolic  logarithm  of  that  number  plus  1,  so  is  the 
initial  to  tlie  mean  pressure,  hence  2  :  1.69314::  20: 
16.9314lbs.  mean  pressure. 

Ans.  2d.  20-^2=10  Ibs.  pressure  at  the  end  of  the 
stroke. 

Ans.  3<£  Work  performed  before  expansion,  1. 
after  expansion,  .69314.  Therefore  1 :  .69314: :  100  : 
69.314  per  cent,  gain  by  cutting  off  at  half  stroke. 

BACK  PRESSURE. 

Inasmuch  as  it  is  impossible  in  practice  to  obtain 
a  perfect  vacuum,  there  is  always  a  certain  amount  of 
steam  in  the  cylinder  opposed  to  the  motion  of  the 
piston,  and  this  is  termed  back  pressure.  Suppose  for 
example,  there  was  in  the  above  instance  4  Ibs.  per 
square  inch  back  pressure,  the  mean  effective,  or  un- 
balanced pressure,  would  be  16.9314—4=12.9314  Ibs., 
and  the  unbalanced  pressure  at  the  end  of  the  stroke 
would  be  10— 4 = 6  Ibs. 

EXAMPLE  2d.  Suppose  the  steam  in  example  1st 
had  been  cut  off  at  a  \  from  commencement  of  stroke, 
what  would  have  been  the  mean  pressure,  pressure  at 
the  end,  and  percentage  of  gain  in  that  case  ?  Also 
the  mean  unbalanced  pressure,  and  unbalanced  pressure 
at  the  end,  the  back  pressure  being  4  Ibs  per  square 
inch? 


BACK    PRESSURE.  17 

Ans.  \st.  4 :  2.38629 : :  20 : 11.93145  Ibs.  mean  pres- 
sure. 

Ans.  2d.  20  -f-  4  =  5  Ibs.  pressure  at  the  end. 

Ans.  3d.  1 : 1.38629  : :  100 : 138.629  per  cent. 

Ans.  till.  11.93145—4=7.93145  mean  unbalanced 
pressure. 

Ans.  5th.  5— 4  =  lib.  unbalanced  pressure  at  the 
end. 

It  is  useless  hereto  multiply  examples ;  those  already 
given  we  consider  sufficient  to  give  the  student  a  clear 
understanding  of  the  manner  in  which  these  calcula- 
tions are  made,  but  we  would  recommend  him  to  make 
a  number  of  others  for  himself,  and  work  them  out  so 
as  to  render  himself  the  more  familiar  and  ready  with 
the  mod^s  operandi. 

We  come  now  to  the  percentage  of  gain  of  fuel  by 
using  steam  expansively.  It  has  been  previously 
shown  that  when  the  steam  is  cut  off  at  one  half,  the 
work  done  before  expansion  takes  place  being  repre- 
sented by  1,  the  work  done  afterwards  is  .69;  the  total 
work  therefore  performed  is  1.69 ;  now  had  not  the 
cut-off  valve  closed  at  all,  the  total  work  performed 
would  have  been  2.  Hence  by  this  operation  we  have 
the  power  of  the  engine  reduced  from  2  to  1.69.  It  is 
therefore  necessary  to  increase  the  initial  pressure  of 
steam  to  make  up  this  decreased  power ;  and  keeping 
in  view  Marriotte's  law,  we  will  let  this  pressure  be 
represented  by  x  •  hence  2  : 1.69 : :  x :  .845  a?,  the  mean 
pressure.  It  is  manifest  that  the  mean  pressure  in  this 
case  must  be  the  same  as  if  the  steam  followed  full 
stroke,  in  order  that  the  powers  may  be  the  same ; 
consequently 

.845  x  —  1  Ib. 

x  =  1.18  Ib.  initial  pressure. 


18  GAIN   BY    EXPANDED    STEAM. 

But  only  half  a  cylinder  full  of  this  steam  is  used  to 
every  full  cylinder  of  the  other,  consequently  the  dif- 
ference between  1.18-^-2  and  1,  equals  the  saving, 
which  is  41  per  cent. 

To  ascertain  the  saving  of  steam  at  any  other  point 
of  cutting  off,  take  the  hyperbolic  log.  of  3,  4,  5,  6, 
<fec.  as  the  cutting-off  point  may  be  i,  £,  i,  £,  <fec.,  and 
proceed  in  the  same  manner,  or,  in  other  words,  divide 
the  whole  length  of  the  stroke  by  the  portion  traveled 
before  the  steam  is  cut  off;  take  the  hyperbolic  log.  of 
the  quotient  and  proceed  as  above. 

The  41  per  cent,  in  the  above  example,  is  the 
saving  in  steam — that  is  to  say,  should  a  steamer, 
using  steam  full  stroke,  perform  a  certain  distance 
in  a  certain  time,  cut  the  steam  off  at  half-stroke, 
and  increase  the  initial  pressure  in  the  ratio  of  1 
to  1.18,  she  would  perform  the  same  distance  in  the 
same  time  with  41  per  cent,  less  steam.  Not  41  per 
cent,  less  coals  pmt  into  the  furnaces,  but  41  per  cent, 
of  that  which  reaches  the  cylinders  minus  the  loss 
from  condensation  due  to  expansion,  i.  e.  that  por- 
tion of  the  fuel  not  combustible,  and  that  portion  pass- 
ing out  of  the  chimney  in  the  shape  of  heat  to  produce 
draft,  together  with  the  loss  from  radiation  and  con- 
densation before  the  steam  reachers  the  cylinders  must 
first  be  deducted.  When  this  is  done  it  will  be  found 
that  the  actual  saving  will  be  reduced  to  less  than  20 
per  cent.,  which  is  about  the  real  saving  of  fuel  in 
practice  cutting  off  at  half  stroke,  and  pro  rata  for  any 
other  point — varying  somewhat  according  to  better  or 
worse  constructions.  Any  engineer  can  satisfy  himself 
on  this  point  by  using  his  steam  with  and  without  ex- 
pansion for  a  sufficient  given  time,  carefully  weighing 
all  the  coals  and  recording  all  the  data. 


EXPANSION    VALVES. 


19 


This  should  not  therefore  be  confounded  with  the 
69  per  cent.,  which  is  theoretically  the  increased  work 
performed  by  the  same  steam  over  what  it  would  have 
performed  had  it  not  been  cut  off  at  all. 

EXPANSION  VALVES. 

There  are  a  variety  of  expansion  valves  and  arrange- 
ments for  cutting  off  steam ;  the  principles  operating 
the  more  important  of  which  we  will  now  proceed  to 
examine.  The  following  diagrams  or  sketches  will 
serve  our  purpose.  We  shall  simply  explain  the  lead- 
ing features  of  each,  in  order  to  give  an  understanding 
of  the  principles  that  govern  them,  leaving  the  student 
to  suggest  for  himself  the  Fig.  i. 

alterations  in  the  mechani- 
cal arrangement  to  adapt 
them  to  different  types 
and  arrangements  of  en- 
gines. 

Figure  1  is  a  diagram 
of  Sickel's  momentarily 
adjustable  cut-off,  in  which 
A,  A,  is  the  steam  valve 
of  the  double  poppet  con- 
struction ;  B,  B,  valve 
stem ;  C,  dash-pot,  filled 
with  water  up  to  the  line  d 
1,  2;  D,  plunger,  fitting 
in  the  dash-pot ;  E,  stuf- 
fing-box, which  is  packed 
air  and  water-tight;  0, 
hole  in  the  bottom  of  the 
plunger  D,  to  allow  the 
water  to  enter  when  the  plunger  strikes  it;  ~b,  a 


20  EXPANSION    VALVES. 

rod  communicating  motion  to  the  wiper  F,  which 
trips  the  valve ;  A,  a  rod  receiving  motion  from  the 
air-pump  beam  or  .any  other  part  having  motion  coin- 
cident with  that  of  the  piston.  The  motion  of  li  is 
communicated  through  the  vertical  rod,  having  c  as  a 
fixed  centre  to  &,  and  thence  to  the  wiper  F.  The 
manner  in  which  this  cut-off  operates  is  this:  The 
valve  stem,  instead  of  being  permanently  attached  to 
the  lifting  rod,  is  secured  to  it  by  a  clutch  and  spring. 
The  valve  is  lifted  by  the  eccentric,  (operating  as  in 
other  cases,)  but  before  it  reaches  its  seat  again,  the 
wiper  F,  which  vibrates  back  and  forth,  strikes  the 
clutch,  and  detaches  the  valve  from  the  lifter;  the 
valve  then,  from  the  action  of  gravity,  would  fall,  and 
strike  its  seat  with  a  heavy  blow ;  to  prevent  which, 
and  allow  the  valve  at  the  same  time  to  fall  quickly, 
it  is  attached  to  the  plunger  D,  working  in  the  dash- 
pot  C.  By  this  arrangement,  before  the  valve  reaches 
its  seat,  the  plunger  D  strikes  upon  the  water  in  the 
lower  part  of  the  dash-pot  C,  which  is  called  the  secon- 
dary reservoir,  and  thereby  allows  the  valve,  to  close 
without  slamming,  the  water  escaping  into  the  cavity  x, 
and  also  around  the  plunger,  into  the  upper  or  primary 
reservoir.  The  plunger  D,  being  hollow,  small  holes 
are  bored  into  it  in  the  vicinity  of  the  line  1,  2,  to 
allow  the  water  to  escape  into  it  also. 

We  see  that  the  cutting-off  is  effected  by  the  wiper 
F  tripping  the  valve ;  the  sooner  therefore  the  valve 
is  tripped,  the  sooner  the  steam  will  be  cut  off.  Now 
the  manner  in  which  this  is  made  an  adjustable  cut-off, 
is  accomplished  by  moving  the  handle  f  backward  or 
forward  on  the  arc  <7,  which  will  move  the  centre  c  to 
one  side  or  the  6ther  of  its  present  position,  the  center  e 
remaining  constantly  fixed,  and  therefore  giving  the 


EXPANSION    VALVES.  21 

wiper  F  a  greater  or  less  distance  to  travel  before 
striking  the  clutch.  By  this  means  the  cutting-off 
point  can  be  varied  for  any  part  of  the  stroke.  The 
handle /can  be  pat  in  such  a  position  that  the  valve 
will  not  be  tripped  at  all,  or  it  can  be  placed  so  that 
the  valve  will  not  lift  at  all,  being  exactly  in  the  ver- 
tical position  when  the  lifter  commences  to  rise.  The 
engine  can  be  thus  stopped  by  this  cut-off.  For  the 
other  end  of  the  cylinder  there  is  another  dash-pot, 
<fec.,  similar  to  the  one  described,  the  wiper  being  ope- 
rated by  a  rod  similar  to  £>,  attached  to  the  center  d. 

Should  there  be  too  much  water  in  the  dash-pot, 
the  valve  will  not  seat  quickly,  but "  hang,"  as  it  is 
technically  termed.  At  a  there  is  a  cock  for  the  pur- 
pose of  supplying  it  with  water.  Attached  to  the 
dash-pot,  there  is  usually  another  cock  or  valve  for 
the  purpose  of  letting  out  any  superfluous  water.  In- 
sufficient water  is  evidenced  by  the  slamming  of  the 
valve. 

This  cut-off  was  formerly  made  without  the  wiper 
F,  there  being  used  instead  a  sliding  cam,  shaped 
something  like  this  <] .  As  the  valve  rose  up,  the 
clutch  struck  thebevel  on  this  cam,  which  forced  the 
clutch  out  of  its  position,  and  allowed  the  valve  to  fall. 
With  this  arrangement,  however,  it  will  be  seen  that 
the  valve  must  trip  while  it  is  rising,  and  as  it  is  at  its 
highest  position  when  the  piston  is  about  half  stroke,  it 
cannot  be  possible  to  cut  off  by  this  mode  longer  than 
half  stroke ;  but  with  the  arrangement  of  the  wiper,  it 
will  be  seen,  inasmuch  as  it  vibrates  back  and  forth, 
that  the  valve  can  be  just  as  well  tripped  on  its  descent 
as  when  rising,  and  this  is  the  reason  why  it  was  sub- 
stituted for  the  cam. 

"  Stevens"'' — The  next  cut-off  that  we  shall  take 


22 


EXPANSION   VALVES. 


FIG.  2. 


into  consideration  is  Stevens's,  a  diagram  of  which  is 
shown  in  figure  2.  A  A  are  the  steam  toes ;  B  B, 
the  steam-lifting  toes ;  D,  rock-shaft  arms ;  C  C,  the 
valves  ;  #,  pin  in  rock-shaft 

arm  for  eccentric  hook.  The 
manner  in  which  this  is 
made  an  adjustable  cut-off, 
is  by  raising  or  lowering  the 
toes  A  A,  thereby  giving 
them  more  or  less  lost  mo- 
tion. In  the  position  in 
which  they  are  shown  in  the 
diagram,  it  will  be  seen  that 
they  will  have  to  travel  a 
considerable  distance  before 
touching  the  toes  B,  B,  and 
as  the  piston  is  in  motion 
during  this  time;  and  the 
steam  valve  closed,  the 
steam  will  be  acting  expan- 
sively. If  the  end  of  the  toes  A  A  be  dropped  lower 
down,  the  steam  will  be  cut  off  shorter ;  if  raised  higher 
up,  longer.  By  dropping  the  toes  down,  however,  we 
diminish  the  lift  of  the  valve,  and  also  alter  the  lead. 
To  retain  the  one,  we  raise  the  pin  a  in  the  rock-shaft 
arm,  and  the  other  we  turn  the  eccentric  a  little  ahead. 
To  alter  the  point  of  cut-off,  therefore,  while  the  engine 
is  in  motion,  so  as  to  cut  off  shorter,  we  have  first  to 
drop  the  toes  A  A,  then  raise  the  pin  «,  and  set  the  ec- 
centric ahead.  To  cut  off  longer,  reverse  the  operation. 
The  number  of  things  required  to  be  altered  in 
changing  the  point  of  cutting-off  is  a  very  great  objec- 
tion to  this  arrangement.  In  practice  it  has  seldom 
been  accomplished  without  stopping  the  engine. 


EXPANSION   VALVES. 


23 


Allen  and  Wells. — This  cut-off  is  represented  by 
sketch,  figure  3.  A,  A,  are  the  exhaust  toes ;  B  B. 
steam  toes ;  C  C  lifting  toes ;  D  D,  the  valves ;  E  E', 


FIG.  3. 


2) 


palls  fitted  to  the  end  of  the  toes  B,  B' ;  F,  rock-shaft 
arm,  which  is  operated  from  the  eccentric  in  the  usual 


24  SLIDE    CUT-OFFS. 

way ;  G  G,  a  cross  arm  secured  to  the  end  of  the  rock- 
shaft  arm ;  a  a,  rollers  on  the  end  of  the  cross-arm  G, ' 
G' ;  II  H,  two  arms  fitted  loosely  on  the  rock-shaft. 
These  arms  receive  their  motion  from  any  part  of  the 
engine  having  motion  nearly  coincident  with  that  of 
the  piston ;  b  &',  rollers  on  these  arms.  This  cut-off 
operates  thus :  The  rock-shaft  is  put  in  motion  by  the 
eccentric.  The  pall  E  resting  upon  the  roller  «,  is 
raised,  and  with  it  the  toe  B,  and  lifter  toe  C ;  but 
after  the  pall  E  is  raised  up  so  as  to  clear  the  roller  I, 
the  pall  E  slides  in  on  top  of  Z>,  which,  having  a  down- 
ward motion,  lowers  the  valve,  while  the  rock-shaft 
arm  continues  to  rise.  The  rollers  b  Z>',  being  attached 
to  the  arms  H  H,  which  having  motion  nearly  coinci- 
dent with  that  of  the  piston,  start  to  go  down  at  nearly 
the  same  time  the  rock-shaft  arm  starts  to.  rise.  Now 
then  by  turning  around  the  right  and  left-hand  screw 
c  c,  the  rollers  b  &',  will  be  set  further  apart,  or  closer 
together,  and  will  therefore  alter  the  time  they  will 
clear  the  end  of  the  pall  E,  and  hence  the  point  of  cut- 
ting off.  To  follow  farther  separate  the  rollers  b  //, 
to  cut  off  shorter,  screw  them  closer  together.  In 
altering  the  point  of  cutting  off  we  have  nothing  to  do 
but  to  turn  around  the  screw  c  c. 

This  cut-off  is  like  "  Sickel's,"  momentarily  adjust- 
able, but  it  cannot,  however,  be  made  to  cut  off  quite 
so  short  as  "  Sickel's." 

SLIDE    CUT-OFFS. 

In  the  use  of  the  ordinary  three-ported  slide-valve, 
or  other  slide-valves  combining,  both  the  steam  and 
exhaust,  the  expansive  principle  can  be  carried  only 
to  a  very  small  extent,  owing  to  the  derangement  of 


SLIDE    CUT-OFFS.  25 

the  exhaust  passages.  Suppose,  for  instance,  that  suffi- 
cient lap  be  given  to  the  steam  side  of  the  valve  to 
cause  the  steam  to  be  shut  off  at  half-stroke,  and  sup- 
pose the  same  amount  of  lap  be  given  also  to  the  ex- 
haust side,  it  is  manifest,  that  when  the  steam  is  shut 
off,  the  exhaust  will  be  shut  off  also,  and  the  pent  up 
steam,  therefore,  having  no  escape,  and  increasing  in 
pressure  as  the  piston  approaches  the  end  of  the  stroke, 
will  act  as  a  serious  retarding  force.  This  arrange- 
ment, therefore,  cannot  operate. 

Now,  then,  suppose  that  we  put  lap  on  the  steam 
side,  as  before,  but  none  on  the  exhaust,  in  which 
event  another  difficulty  equally  great  presents  itself. 
It  is  this : 

Supposing  the  valve,  Fig.  4,  to  have  neither  lap 
nor  lead,  when  the  end  a  arrives  at  a',  steam  will  just 
begin  to  be  admitted  into  the  cylinder,  but  the  point 


Fio.  4. 


7;,  at  the  same  time,  will  have  arrived  at  the  point  Z>', 
and  steam  just  begin  also  to  exhaust ;  now,  then,  let 
half  an  inch  be  added  to  each  end  of  the  valve  at  a 
and  £,  when  the  valve  begins  to  open  to  steam  in  this 
case,  a,  instead  of  being  at  «',  will  be  half  an  inch  past 
it ;  and,  as  there  has  been  no  lap  added  to  the  exhaust 
side,  I  will  be  half  an  inch  past  V,  so  that  the  exhaust 


26 


SLIDE    CUT-OFFS. 


must  have  opened  considerably  before  the  piston  ar- 
rived at  the  end  of  the  stroke  ;  hence,  in  this  case,  we 
exhaust  too  soon. 

All  we  can  do,  therefore,  in  practice,  is  to  strike  a 
mean  between  these  evils ;  that  is  to  say,  when  we  add 
lap  to  the  steam  side,  add  lap  also  to  the  exhaust  side, 
but  not  so  much  so  that  we  open  the  exhaust  before 
the  piston  arrives  at  the  end,  and  close  it  again  before 
it  reaches  the  other  end.  The  shortest  this  kind  of 
valve  can  be  made  to  cut-off  to  advantage  in  practice, 
is  considered  about  J-  from  commencement  of  stroke ; 
but  even  this  we  consider  most  too  short  for  beneficial 
working  of  large  engines. 

Owing  to  these  confined  limits,  the  beneficial  re- 
sults obtainable  from  the  expansive  principle  by  this 
arrangement  is  very  small,  which  has  led  to  the  adop- 
tion of  an  independent  slide  cut-off  valve,  situated  on  a 
separate  face,  back  of  the  steam  valve,  as  shown  in 


FIG.  5. 


Fig.  5,  in  which  a'  is  the  steam,  and  b  b  the  cut-off 
valve.  The  valve  a  having  only  sufficient  lap  to  cover 
the  ports  a!  a'  fairly,  when  it  is  in  the  middle  of 
the  stroke,  operates  as  in  other  cases,  but  the  lap 
on  b  b  can  be  made  to  any  required  extent,  so  that 


SLIDE   CUT-OFFS.  27 

during  a  large  part  of  the  stroke  the  ports  V  I' 
are  closed,  preventing  further  access  of  steam  to  the 
cylinder,  notwithstanding  the  steam  valve  itself  is 
open.  The  valve  b  b  is  operated  by  an  independent 
eccentric,  through  the  valve  stem  E.  In  the  position 
shown  in  the  figure  the  steam  is  cut  off  about  half 
stroke:  d'  shows  another  opening  covered  with  the 
valve  d,  having  a  stem  c  sliding  loosely  through  the 
valve  b  b  /  the  other  end  of  the  stem  passing  through 
the  chest,  has  a  handle  attached  to  it  for  the  purpose 
of  moving  the  valve  d,  in  order  to  open  the  port  </, 
when  the  engine  is  stopped.  This  is  necessary,  for  the 
reason  that  the  engine  may  stop  when  the  valve  1)  b  is 
in  such  a  position  as  to  prevent  the  steam  from  enter- 
ing to  the  steam  valve  #,  and  the  engine  could  not, 
therefore,  be  started.  In  the  figure,  the  cut-off  valve 
has  but  two  ports  for  the  admission  of  steam,  but  any 
number  of  ports  can  be  made — the  more  numerous, 
the  less  stroke  will  be  required  to  get  the  necessary 
opening.  This  is  what  is  termed  the  gridiron  valve, 
from  the  resemblance  it  bears  to  that  very  use/id  in- 
strument. 

After  this  valve  is  once  made,  the  point  of  cutting 
off  usually  remains  fixed,  but  it  can,  however,  be  varied 


FIG.  6. 


counters 
Kcnrrnic 


within  narrow  limits  by  altering  the  stroke  of. the 
valve.  Thus,  in  Fig.  6,  supposing  the  end  of  the  valve 
stem  to  be  raised  from  a  to  5,  the  valve,  instead  of 


28  SLIDE    CUT-OFFS. 

being  closed,  as  shown,  will  be  open  the  distance  b  c, 
and  will  therefore  have  that  much  additional  to  travel 
before  the  steam  is  cut  off;  hence,  by  increasing  the 
travel  of  the  valve  we  increase  the  point  of  cutting  off, 
and  conversely,  supposing  the  pin  a  had  been  lowered 
in  the  rock-shaft  arm  the  distance  a  e,  equal  to  a  Z», 
the  ports,  instead  of  being  closed,  as  shown,  would 
be  closed  the  distance  b  c;  the  steam,  therefore,  would 
be  cut  off  sooner.  But  by  altering  the  point  of  cutting 
off  we  also  alter  the  lead  of  the  valve  ;  for,  taking  the 
case  in  which  we  increased  the  travel  of  the  valve,  we 
see  that  when  it  would  have  been  closed  with  the 
original  lead,  it  lacked  the  distance  b  c.  If  its  travel 
had  been  reduced,  it  would  have  lacked  that  much  of 
being  open.  To  obviate  this,  whenever  the  travel  of 
the  valve  is  altered,  the  eccentric  should  also  be  altered, 
so  as  to  retain  the  original  lead. 

If  the  travel  of  the  valve  be  made  too  great,  the 
valve  d  will  pass  entirely  over  the  port  d',  and  gradu- 
ally close  ^,  unless  they  be  set  some  distance  apart. 
If  the  travel  be  made  too  small,  the  steam  will  be  shut 
off,  and  the  motion  of  the  eccentric  being  reversed 
long  before  the  piston  arrives  at  the  end  of  the  stroke, 
steam  will  be  admitted  to  it  again  before  the  steam 
valve  closes. 

From  the  above  facts,  and  the  figure  before  us,  we 
draw  the  following  general  conclusions  in  reference  to 
this  kind  of  slide  cut-off  valves : 

That,  with  a  given  amount  of  lap,  the  cutting  off 
point  can  be  varied  from  the  longest  point  of  cutting 
off  allowable  by  said  lap,  to  a  certain  point  within  the 
stroke,  by  reducing  the  stroke  of  the  valve  and  alter- 
ing the  eccentric  so  as  to  retain  the  original  lead.  If 
the  stroke  be  reduced  beyond  this,  steam  will  be  shut 


SLIDE    CUT-OFFS. 


29 


off  and  given  to  the  piston  again  before  it  arrives  at 
the  end  of  the  stroke.  In  practice,  this  variation  will 
not  amount  to  more  than  from  about  J-  to  f  of  the 
stroke. 

In  altering  the  stroke  of  the  valve,  the  slot  through 
which  the  pin  a  moves  should  be  an  arc  of  a  circle, 
struck  with  a  radius  equal  to  the  length  of  the  link  d  a, 
and  with  d  as  a  centre. 

With  equal  leads,  the  cutting  off  point  cannot  be 
effected  equally  on  both  ends  of  the  cylinder  with  a 
slide  valve,  owing  to  the  connecting  rod  acting  out  of 
parallelism,  or,  in  other  words,  owing  to  the  crank  not 
being  at  90°  when  the  piston  is  half  way.  The  shorter 
the  connecting  rod,  the  greater  the  discrepancy. 


FIG.  6j. 


CONNECTSTO  CONDENSER 


Fig.  (>£,  is  an  arrangement  of  cut-off  valve  as  con- 
structed by  Messrs.  Merrick  &  Son,  of  Philadelphia, 
in  1855,  for  the  U.  S.  Steam  Frigate  "  Wabash." 

In  consequence  of  the  satisfactory  manner  in  which 
it  worked  on  board  that  vessel;  its  simplicity,  and 
easy  adjustment  for  cutting  off  at  any  portion  of  the 


30  OTHER   KIND    OF   VALVES. 

stroke  likely  to  be  required,  it  lias  been  applied  to 
nearly  all  the  U.  S.  Screw  ships  recently  constructed, 
as  also  to  a  number  of  other  engines.  C  is  the  steam 
valve ;  D  D  are  the  cut-off  valves,  attached  to  the  valve 
stem  E  by  right  and  left  hand  screws  working  in  nuts 
let  into  the  valves  ;  F  F,  rings  in  the  steam  chest  cover, 
fitting  close  down  on  the  back  of  the  main  steam  valve, 
enclosing  the  space  Gr,  which  is  connected  to  the  con- 
denser by  the  pipe  H,  for  the  purpose  of  balancing 
the  valve. 

This  cut-off  can  be  worked  by  a  separate  eccentric, 
or  from  any  part  of  the  engine  having  a  motion  coin- 
cident with  that  of  the  piston. 

To  alter  the  point  of  cutting-off,  a  wheel  is  on  the 
end  of  the  valve  stem  E,  which,  if  turned  in  one  direc- 
tion, will  draw  the  valves  closer  together,  and  the 
openings  will  not  be  closed  so  soon,  consequently  the 
steam  will  follow  the  piston  farther,  i.  e.  cut  off  longer. 
To  cut  off  shorter,  the  operation  is  reversed. 

OTHER    KIND    OF   VALVES. 

Having  explained  the  principles  of  the  leading  cut- 
offs, we  will  now  take  a  glance  at  some  of  the  most 
prominent  steam  and  exhaust  valves  now  in  use ;  but, 
inasmuch  as  the  student  is  supposed  to  understand  the 
leading  features  of  most  of  these,  we  will  not  devote 
much  time  to  this  part  of  our  study. 

Figure  Y  is  a  diagram  of  a  double  poppet  valve, 
in  which  the  rectangular  space,  abed  is  the  open- 
ing to  the  cylinder ;  A  B,  the  steam  valves,  and  C  D, 
the  exhaust  valves.  The  object  of  this  arrangement  is 
to  make  the  valve  a  balance  valve.  Thus  the  steam 
acting  on  the  top  of  A  and  bottom  of  B,  if  they  were 


OTHER   KIND   OF   VALVES. 


31 


of  equal  size,  an  equilibrium  would  be  established,  but 
the  valve  B  is  made  just  small  enough  to  slip  througli 


Fio.  7. 


FIG.  8. 


the  upper  seat,  so  that  the  difference  in  area  serves  to 
keep  the  valves  fairly  in  their  seats.*  On  the  exhaust 
side  the  reverse  is  the  case.  The  steam  acts  under  C, 
and  on  top  of  D,  the  lower  valve  D  is  usually  made 
the  larger.  In  order  to  get  D  into  its  place,  the  upper 
seat  is  either  made  removable  by  being  secured  in  its 
place  by  tap-bolts,  or  a  hand-hole  is  cut  in  the  side  of 
the  steam-chest,  or,  in  some  cases,  it  is  passed  in  through 
the  cylinder  nozzle. 

Figure  8  is  a  diagram  of  the  single  poppet  valve, 
in  which  A  is  the  steam  valve,  and  B,  the  exhaust. 
With  these  kinds  of 
valves  we  see  that  we 
require  considerable 
power  to  operate 
them  by  hand,  as  we 
have  the  full  pressure 
of  steam  on  the  back 
of  A,  and  also  the 
exhaust  on  B;  but 
when  the  engine  is  hooked  on  the  pressure  is  in  part 
balanced.  On  the  steam  valve  this  is  occasioned  at 
the  time  the  valve  is  opened,  by  the  exhaust  valve 

*  In  some  cases,  tha  area*  of  the  valves  are  equal,  and  they  are  seated  by  their  own  weight. 


32 


OTHER    KIND    OF    VALVES. 


being  closed  before  the  piston  arrives  at  the  end  of  the 
stroke,  producing  the  pressure  called  cushion.  And 
on  the  exhaust  valve  the  pressure  is  reduced  (at  the 
time  the  valve  is  opened)  by  expansion.  In  some 
cases  this  pressure  is  but  little  above  that  in  the  con- 
denser. It  is  therefore  obvious  that  these  valves  can 
be  made  to  work  with  but  little  power  from  the  engine. 
They  also  have  the  advantage  of  being  easily  made 
tight  and  occupying  but  little  room. 

The  disadvantage  of  working  by  hand,  however, 
led  to  the  adoption  of  the  double  poppet  valve,  the 
single  poppet  being  the  earlier  invention.  The  double 
poppet  valve  is  the  one  now  almost  universally  used  in 
American  low-pressure  river,  or  marine  paddle-wheel 
engines. 

Figure  9  is  a  representation  of  what  is  termed 
"  Hornblower's  "  valve,  in  which  a  a  b  b  are  the  valve 


FIG.  9. 


seats ;  A  A,  the  valve ;  B,  one  of  a  number  of  cross- 
bars secured  to  the  top  of  the  valve,  to  which  the 


OTIIER    KIND    OF    VALVES. 


valve  stem  is  attached.  From  the  figure  it  will  be 
seen  that  the  only  surface  the  steam  has  to  act  upon 
to  keep  the  valve  in  its  seat,  is  the  upper  edge,  c  <",  of 
the  valve  ;  it  is  therefore  an  equilibrium  valve. 

Figure  10  is  what  is  termed   a  box  valve;    a  <( 
are  the  parts  communicating    with   the    cylinder;  &, 


Fio.  10. 


steam-pipe ;  <?,  the  exhaust ;  A  A,  the  valve  having  an 
opening  through  its  center  communicating  with  the 
exhaust,  c  ;  d  d,  packing.  An  inspection  of  the  figure 
will  show  the  operation  of  the  valve.  The  object  of 
this  kind  of  valve  is  also  to  establish  an  equilibrium. 
Figure  11  is  a  longitudinal  section,  and  figure  12 


FIG.  11 


a  top  view  of  what  is  termed  the  equilibrium  slide. 


34 


OTHER    KIND    OF    VALVES. 


Fie.  12- 


This  valve  has  a  ring,  A  A,  on  the  back  of  it,  which 
being  made  steam  tight,  the 
pressure  is  taken  off  the  space 
enclosed  by  the  ring.  The  pres- 
sure is  taken  off  the  back  of 
nearly  all  the  valves  of  large  en- 
gines now-a-days,  fitted  with  the 
short  slide,  either  in  this  way  or 
by  having  the  ring  secured  to 
the  top  of  the  chest,  and  the  valve  sliding  under  it. 

Figure  13  shows    a   slide  valve  A  A  A,  having 
openings  b  b  through  it  for  the  admission  of  steam ; 

FIG.  13. 


a  a  a  is  another  valve  sliding  on  the  back  of  the 
valve  A  A  A;  a  a  a  is  the  cut-off,  which  operates 
thus :  The  valve  A  A  A  being  put  in  motion,  and 
the  cut-off  valve  lying  loosely  on  its  back,  is  carried 
with  it  until  the  end  of  the  valve  «,  «,  «,  strikes  the 
steam  chest,  when  its  motion  is  arrested,  while  the 
steam  valve  continues  to  move,  the  result  is  the  clos- 
ing the  opening  &,  and  the  cutting  off  the  steam.  The 
sooner,  therefore,  the  slide  a  a  a  strikes  the  chest,  the 
sooner  the  steam  is  cut  off.  The  point  of  cutting-off 
can  be  varied  by  having  a  screw  running  through  the 


OTHER    KIND    OF   VALVES. 


35 


ft! 


chest,  which  can  be  moved  further  in  or  out  and 
against  which  the  valve  a  a  a  strikes. 
With  this  arrangement  it  will  be  seen 
that  the  cut-off  must  close  at  further- 
est  a  little  before  the  piston  arrives  at 
half  stroke,  or  not  close  at  all.  This 
cut-off  is  applicable  to  horizontal  sta- 
tionary engines. 

Fig.  14  is  a  piston  valve,  in  which 
a  a'  are  the  openings  into  the  cylin- 
der ;  C,  exhaust  opening ;  A  B  D  E 
the  valve  packed  at  b  c  d  e  with 
rings  or  other  packing.  In  the  position 
shown  in  the  figure,  steam  is  being 
exhausted  through  the  openings  a' 
and  C  into  the  condenser,  while  steam 
is  being  admitted  into  the  opposite 
end  of  the  cylinder  through  the  open- 
ing a.  When  the  valve  has  its  full 
throw  in  the  opposite  direction,  steam 
will  be  admitted  through  the  opening 
a'  while  it  is  being  exhausted  through 
«,  and  the  opening  F  F  througji  the 
valve  and  through  C  into  the  con- 
denser. 


36  OTHER    KIND    OF    VALVES. 

Figure  15  shows  the  long  D  slide,  with  the  full 


FIG.  15. 


opening  for  steam  under  the  piston;   Fig.   16,  same 


FIG.  16. 


valve  showing  full  opening  for  steam  on  top  of  the 
piston  ;  Fig.  IT,  longitudinal  section  of  the  valve  alone, 


FIG.  18. 


FIG.  17. 


and  Fig.  18,  cross  section  of  the  same.  A  is  the  steam 
pipe,  B,  the  exhaust,  C,  packing  to  keep  the  steam  and 
exhaust  separate,  steam  "being  admitted  into  the  chest 
or  valve  casing  at  A,  fills  the  vacant  space  under  and 
around  the  valve,  but  cannot  escape  past  the  ends 


OTHER    KIND    OF    VALVES. 


FIG.  19 


Fio.  20. 


owing  to  the  packing  C  C ;  and,  when,  the  valve  is 
placed  in  the  position  shown  in  Fig.  15,  steam  is  ad- 
mitted under  the  piston  in  the  direction  shown  by  the 
arrows,  at  the  same  time  that  it  is  exhausted  through 
the  upper  opening,  and — the  valve  being  hollow— 
through  it  and  pipe  B  into  the  condenser.  When  the 
valve  is  moved  in  the  opposite  direction,  steam  is  ad- 
mitted above  the  piston  in  the  direction  shown  by  the 
arrows  in  Fig.  16,  and  exhausted 
through  the  lower  opening  directly 
through  the  pipe  B  to  the  condenser. 

This  style  of  valve  is  in  extensive 
use  on  English  marine,  and  other 
engines.  The  objection  to  it  is  the 
friction,  requiring  several  men  to 
work  the  starting  bar  when  the  en- 
gine is  operated  by  hand. 

Fig.  19  is  a  longitudinal  section 
of  the  short  D  slide,  and  Fig.  20,  an 
end  view  of  the  same.  A  A'  are  the 
openings  into  the  cylinder ;  B  B,  the 
communications  to  the  condenser ;  C, 
steam  pipe.  In  the  position  shown 
in  the  figure,  steam  is  being  admitted 
through  A  into  the  cylinder,  and  ex- 
hausted through  A'  into  the  con- 
denser, c  c  is  packing  on  the  back  of 
the  valve. 


(o, 


38 


OTHER    KIND    OF    VALVES. 


FIG.  21. 


FIG.  22. 


VALVE 


FACE 


Figure  21  is  a  view  of  the  Worthington  pump 

steam  valve;  figure 
22,  the  valve  face, 
and  figure  23,  the 
valve  seat.  The  fig- 
ures explain  them- 
selves. In  the  or- 
dinary slide  valve, 
when  it  is  moved 
in  one  direction, 
steam  is  given  to 
the  piston  in  the 
same  direction,  but 
the  object  of  this 
valve,  as  invented 
by  H.  E.  Worth- 
ington, of  N.  York, 
is  to  cause  it,  when 
moved  in  one  direc- 
tion, to  give  steam 
to  the  piston  in  the 
opposite  direction. 
The  valve  being  operated  by  an  arm  projecting  from 
the  piston  rod,  which  strikes  collars  on  the  valve  stem, 
renders  it  necessary  that  when  the  valve  is  moved  in 
one  direction,  steam  should  be  given  to  the  piston  in 
the  opposite  direction,  in  order  to  reverse  its  motion  ; 
by  this  arrangement  the  intervention  of  levers  is  un- 
necessary, as  the  end  is  accomplished  direct. 


FIG.  23. 


VALVE 


SEAT 


THE   PITTSBUKO    CAM. 


39 


FIG.  24. 


FIG.  25. 


The  Pittsburg  Cam. — Figures  24,  25,  and  26,  show 

different  forms  of  this 
cam.  Like  letters  refer 
to  like  parts.  A  B  C  D 
is  a  yoke  fitting  over  the 
cam  a  b  c  ;  E  is  a  rod 
attached  to  the  valve 
stem.  F,  main  shaft  of 
the  engine  to  which  the 
cam  is  secured.  It  will 
be  seen  that  by  the  revo- 
lution of  the  cam  a  b  c, 
within  the  yoke  ABC 
D,  the  rod  E  will  be 
paused  to  move  back  and 
forth,  and  thereby  open 
and  shut  the  valve. 

Fig.  24  is  a  cam  made 
to  cut  off  at  half  stroke ; 
figure  26,  £  stroke,  and 
figure  25  follows  full 
stroke.  The  manner  in 
which  these  cams  are 
laid  off  is  this.  From 
the  centre  F,  with  a  ra- 
dius dependent  upon  the 
stroke  of  the  valve,  de- 
scribe a  circle,  as  shown 

_ _/T\  _  k partly  in  dots  and  partly 

in  full  lines  in  the  fig- 
ures ;  divide  this  circle 
into  any  convenient  even 
number  of  equal  parts, 
say  sixteen ;  then,  supposing  we  wish  to  cut  off  at  half 


40  OTHER    KIND    OF    VALVES. 

stroke,  taking  figure  24,  place  one  foot  of  the  dividers 
having  a  radius  equal  to  the  diameter  of  the  circle  at 
C,  and  describe  the  arc  terminating  at  <5,  then  move 
the  foot  of  the  dividers  from  c  to  #,  and  describe 
another  arc  terminating  also  at  b  •  then,  with  the  same 
radius,  and  I  as  a  centre,  describe  the  arc  a  c;  the 
figure  thus  enclosed  will  be  the  required  cam.  It  will 
be  observed  that,  while  the  cam  is  traveling  the  dis- 
tance a  1 — that  being  an  arc  of  a  true  circle — no  mo- 
tion can  be  given  to  the  valve,  but  while  it  travels 
from  1  to  2  the  valve  is  opened  and  shut.  Now,  then, 
inasmuch  as  the  piston  moves  from  one  end  of  the  cyl- 
inder to  the  other  for  each  semi-revolution  of  the  cam, 
and  inasmuch  as  the  distance  from  a  to  1  is  the  same 
as  from  1  to  2,  the  valve  remains  necessarily  closed 
during  one-half  of  the  stroke. 

In  figure  25,  as  no  part  of  the  outline  of  the  cam  is 
concentric  to  the  shaft  F,  the  valve  must  be  in  motion, 
all  the  time  the  cam  is  in  motion.  In  figure  26,  as 
three-quarters  of  the  semi-periphery  of  the  cam  is  con- 
centric to  the  shaft  F,  the  valve  will  remain  closed 
during  three-quarters  of  the  stroke.  Instead  of  making 
the  points  Z>  sharp,  as  shown  in  the  figures,  they  can  be 
turned  off,  and,  to  retain  the  same  dimensions  on  the 
cam,  an  equal  amount  added  to  the  arc  a  c.  Thus, 
taking  figure  25,  suppose  we  cut  off  the  point  of  the 
cam  to  cc  y,  and  increase  the  lower  extremity  to  H  I, 
this  will  not  alter  the  point  of  cutting  off,  but  it  re- 
duces the  travel  of  the  valve,  and  has  the  effect  of 
keeping  the  valve  stationary  when  wide  open,  while 
the  cam  travels  through  the  arc  cc  y. 


CHAPTER  II. 

THE   INDICATOR    AND    INDICATOR    DIAGRAMS. 

THE  steam  engine  indicator  is  an  instrument  used 
tor  the  purpose  of  exhibiting  the  performance  of  the 
steam  engine.  By  its  application  to  the  steam  cylinder 
we  can  ascertain  the  following  particulars:  Whether 
the  valves  are  properly  constructed  and  set ;  steam  and 
exhaust  passages  of  the  right  size ;  whether  the  piston 
or  valves  leak;  the  amount  of  vacuum  or  back  pres- 
sure, and  pressure  of  steam  -upon  the  piston ;  the  power 
of  the  engine ;  power  required  to  overcome  its  friction, 
and  also  to  work  any  machinery  attached  to  the  same, 
<fcc.  In  truth,  it  is  the  stethoscope  of  the  physician, 
revealing  the  internal  working  of  the  engine. 

The  following  description  of  the  instrument  and  cut, 
Fig.  27,  we  take  from  Paul  Stillman's  Treatise  on  the 
Indicator.  The  cut  shows  the  style  manufactured  at 
the  Novelty  Iron  Works,  New  York  city : 

A  is  a  brass  case  enclosing  a  cylinder,  into  which  a 
piston  is  nicely  fitted.  To  the  piston-rod  a  spiral 
spring  is  attached  to  resist  the  steam  and  vacuum 
when  acting  against  it.  B  is  a  pencil  attached  to  the 
piston  rod.  C  is  an  arm  attached  to  the  case,  and  sup- 
porting a  cylinder  D,  which  may  be  caused  to  rotate 
back  and  forth — a  part  of  a  revolution  in  one  direc- 
tion, by  means  of  a  line  or  cord  0,  attached  to  a  suit- 
able part  of  the  engine — and  in  the  other  by  means  of 


42 


THE   INDICATOR    AND   INDICATOR   DIAGRAMS 


FIG.  2T. 


a  strong  watch  spring  within  the  cylinder  D. 
this  cylinder  is  to  be  wound  a 
paper,  upon  which  a  diagram 
will  be  made,  by  the  combined 
action  of  the  piston  and  paper 
cylinder,  representing,  by  its 
area,  the  power  exerted  on  one 
side  of  the  piston  during  the 
whole  revolution  of  the  engine, 
//are  springs  to  secure  the  paper 
to  the  cylinder ;  g  is  a  scale 
divided  into  parts  corresponding 
to  the  pounds  of  pressure  on  the 
square  inch.  These  divisions, 
for  convenience  of  measuring  the 
diagrams  with  a  common  rule, 
are  generally  made  in  some  re- 
gular parts  of  an  inch,  as  8ths, 
lOths,  12ths,  20ths,  30ths;  h  is 
a  cock  by  means  of  and  through 
which  it  is  connected  with  the 
engine  cylinder. 


Outside 


HOW  TO  ATTACH  THE  INDICATOR. 

Into  whatever  part  of  the 
engine  it  may  be  desired  to  ap- 
ply the  indicator,  there  must 
first  be  inserted  a  small  stop-cock,  with  a  socket  to 
receive  the  one  connected  with  the  indicator.  The 
instrument  is  to  be  set  into  this  in  such  a  position  that 
the  line  attached  to  the  paper  cylinder  shall  lead 
through  or  over  the  guide  pulley  toward  the  place 
whence  it  is  to  receive  its  motion.  An  extension  of 
this  line  should  be  connected  with  some  part  of  the 


THE   INDICATOR    AND    INDICATOR    DIAGRAMS.  43 

engine,  the  motion  of  which  is  coincident  with  that  of 
the  piston,  and  which  would  give  the  paper  cylinder  a 
motion  of  about  three-fourths  of  a  revolution.  If  the 
engine  is  of  the  construction  denominated  beam  or 
lever  engine,  and  is  provided  with  a  "  parallel  motion  " 
the  parallel  bar,  or  a  pulley  on  the  radius  shaft,  fur- 
nishes the  proper  motion;  if  otherwise,  the  beam 
centre  may  be  resorted  to.  In  the  kind  denominated 
square  engines,  the  centre  of  the  air  pump  gives  it.  In 
horizontal  and  vertical  direct  acting  engines,  it  will 
frequently  be  found  necessary  to  erect  a  temporary 
rock-shaft,  or  lever,  connected  with  the  cross-head. 
Particular  care  should  be  taken,  when  the  power  of  the 
engine  is  to  be  estimated,  that  the  motion  communi- 
cated be  perfectly  coincident  with  that  of  the  piston. 

In  nearly  all  forms  of  the  steam  engine,  the  proper 
motion  may  be  obtained  by  attaching  a  line  to  the 
cross-head,  and  passing  it  over  a  delicately  constructed 
pulley,  to  the  axis  of  which  should  be  attached  a 
smaller  one,  from  which  a  line  shall  connect  with  the 
indicator.  The  proportional  sizes  of  the  two  pulleys, 
of  course,  should  be  as  the  distance  traveled  by  the 
piston  to  the  length  of  motion  given  to  the  paper 
cylinder  of  the  indicator.  It  will  be  necessary  to  at- 
tach a  strong  spring  to  the  axis  of  these  pulleys,  to 
produce  the  reverse  motion  promptly.  In  an  oscillat- 
ing engine,  it  will  be  necessary  that  the  indicator,  with 
its  fixtures,  should  be  attached  to  the  cylinder. 

As  the  paper  cylinder  cannot  make  more  than 
about  three-fourths  of  a  revolution  without  disturbing 
the  point  of  the  pencil,  it  will  be  seen  that  the  line 
communicating  the  motion  must  be  of  a  definite  length. 
It  also  requires  to  be  readily  connected  and  discon- 
nected. • 


44 


THE   INDICATOR   AND   INDICATOR   DIAGRAMS. 


The  indicator  having  been  attached  to  the  steam 
cylinder,  the  paper  secured  smoothly  on  the  cylinder 
D,  figure  27,  and  the  length  of  the  line  e  being  ad- 
justed so  that  by  the  vibration  of  D  it  does  not  strike 
the  stops,  we  will  proceed  to  take  a  diagram,  first 
taking  care  to  see  that  the  paper  cylinder  D  is  so  fixed 
that  the  springs  ff  do  not  come  in  contact  with  the 
pencil  B.  The  pencil  B  being  adjusted  so  that  it 
touches  lightly  on  the  paper,  throw  it  back  and  attach 
the  hook  on  the  line  E  to  the  line  receiving  motion 
from  the  engine ;  then  open  the  cock  h,  and  allow  the 
piston  to  work  up  and  down  several  times,  in  order  to 
heat  and  expand  all  the  parts  of  the  instrument.  This 
being  accomplished,  turn  the  pencil  on  and  take  the 
diagram.  Shut  off  the  cock  /*,  and  apply  the  pencil 
again  to  the  paper,  and  it  will  describe  the  atmospheric 
line. 

Figure  28  is  a  diagram  taken  from  the  U.  S.  S. 


FIG.  28 


10)  204.5 


20.45  Ibs.  mean  unbalanced  pressure. 


THE    INDICATOR    AND    INDICATOR    DIAGRAMS.  45 

Frigate  "  Powhatan,"  fitted  with  the  double-poppet 
balanced  valves,  and  Sickels'  cut-off,  on  the  15th  of 
January,  1854,  while  on  the  passage  from  Hong  Kong, 
China,  to  the  Loo-Choo  Islands.  At  #,  the  piston  of 
the  indicator  being  at  the  bottom  of  its  stroke,  steam 
is  admitted,  forcing  it  up  to  b  ;  at  b  the  cylinder  upon 
which  the  paper  is  wound — having  motion  coincident 
with  that  of  the  steam  piston — starts  to  turn,  describ- 
ing the  line  b  <?/  at  c  the  expansion  valve  closes,  and 
the  pressure  therefore  gradually  falls  to  d,  where  the 
exhaust  valve  opens  and  the  pressure  falls  suddenly  to 
e  •  the  steam  piston  now  starts  on  the  return  stroke, 
and  the  spring  within  the  cylinder  D,  fig.  27,  forces  it 
back  to  the  beginning  a  of  the  diagram.  The  line 
from  a  to  b  is  called  the  receiving  line  ;  from  b  to  c  the 
steam  line  ;  from  c  to  d,  the  expansion  line  ;  d  to  0,  the 
exhaust ;  e  to  «,  the  vacuum  line.  The  numbers  in 
the  vertical  column  on  the  right-hand  side  of  the  figure, 
are  the  pounds  pressure  ;  14.7  is  the  true  vacuum  line, 
0,  the  atmospheric  line,  and  14,  the  initial  pressure  of 
steam  above  the  atmosphere.  The  figures  along  the 
top  line  are  the  feet  in  length  of  the  cylinder.  It  will 
be  seen  that  the  cut-off  valve  closed  when  the  piston 
had  traveled  a  very  little  beyond  half  stroke.  The 
rounding  at  d  and  a  is  the  lead  and  cushion  on  the 
exhaust.  That  is  to  say,  the  exhaust  valve  opened  at 
d,  before  the  piston  arrived  at  the  end  of  the  stroke, 
and  it  also  closed  again  at  «,  before  the  piston  reached 
the  other  end  of  the  stroke.  Had  there  been  no  lead 
both  of  these  corners  would  have  been  well  defined. 

In  order  to  calculate  the  power  of  the  engine,  the 
mean  pressure  on  the  piston  must  be  known,  and  from 
no  source  but  the  indicator  can  it  be  accurately  ascer- 
tained. The  manner  of  arriving  at  this  is  simply  by 


INDICATOR   DIAGRAMS. 


taking  the  total  pressure  at  different  points  and  adding 
them  up  and  taking  the  mean,  as  shown  in  fig.  28. 

Figure  28  is  what  would  be  termed  among  en- 
gineers a  good  diagram;  so  is  also  figure  29,  which  we 
will  take  for  a  further  elucidation  of  the  subject. 


Steam, 10 

Vacuum, 27 

Hot  well, 106  Fahr. 

Revolutions,....       9.5 
Throttle, 8. 


FIG.  29. 

"  Powhatan  "  stb.  cylinder,  bottom 

NOT.  T,  1855,  10  A.  M. 
One  engine  and  one  wheel  in 
operation. 
Smooth  sea. 


It  appears  from  this  diagram,  however,  that  the 
piston  of  the  indicator  worked  rather  tightly,  which 
occasioned  it  to  stick  a  little  in  some  places,  as  is  evi- 
denced by  the  steps  in  the  expansion  line,  and  also  at 
a  f>  in  the  vacuum  line.  If  the  piston  of  the  indicator 
become  much  scratched,  similar  effects  will  be  produced. 
Great  care  should,  therefore,  be  taken  in  its  use,  to  see 
that  neither  the  piston  works  too  tightly  nor  too  loose- 
ly ;  for  on  the  one  hand  it  will  stick,  and  thereby  pro- 
duce an  imperfect  outline,  and  on  the  other  hand  will 
produce  the  same  effect  by  exhibiting  false  vacuum  and 
expansion  lines. 


INDICATOR    DIAGRAMS.  47 

Should  figure  29,  instead  of  being  as  shown  in  full 
lines,  have  the  lower  right  hand  corner  cut  off  as  shown 
in  dots  at  c  d,  the  defect  would  have  been  that  the  ex- 
haust valve  closed  too  soon — at  c  instead  of  e — occa- 
sioning excessive  cushioning.  With  some  engines, 
however,  a  large  amount  of  cushioning  is  necessary  to 
prevent  them  from  thumping  on  the  centres. 

Had  the  upper  right-hand  corner  been  rounding,  as 
shown  by  the  dotted  line  f  g,  the  defect  would  have 
been  that  the  steam  valve  opened  too  late.  Had  the 
exhaust  corner  been  cut  off,  as  shown  by  the  dotted 
lines  li  i,  the  exhaust  valve  would  have  opened  too 
soon  ;  but  had  it  been  in  the  form  shown  by  the  dot- 
ted line  &  £,  it  would  have  opened  too  late,  and  after  it 
did  commence  to  open,  would  move  with  too  slow  a 
velocity,  preventing  the  free  escape  of  steam,  or  the 
exhaust  passages  would  have  been  too  small,  which 
would  produce  a  similar  effect  to  the  valve  opening  too 
slowly.  Had  the  steam  line,  instead  of  being  parallel 
to  the  atmospheric  line,  fallen  down  in  the  direction 
in,  n,  it  would  have  shown  that  the  throttle  was  par- 
tially closed,  or  the  steam  passages  too  small,  prevent- 
ing the  full  flow  of  steam  into  the  cylinder. 

Should  there  be  excessive  lead  given  to  the  steam 
valve,  the  line  d  m,  instead  of  being  at  right  angles  to 
the  atmospheric  line,  will  have  the  top  inclined  to  the 
right  as  from  L  to  M. 

In  taking  a  diagram  for  the  purpose  of  estimating 
the  power  of  the  engine  only,  the  atmospheric  line  is 
not  necessary ;  but  in  order  to  ascertain  the  vacuum 
it  cannot  be  dispensed  with,  unless  the  indicator  piston 
be  forced  down  to  the  perfect  vacuum  and  held  there 
until  that  line  be  described. 

Tn  a  diagram  taken  from  a  non-condensing  engine, 
4 


48  INDICATOR   DIAGRAMS. 

the  atmospheric  line  will  of  course  be  entirely  below 
it,  owing  to  the  back  pressure  occasioned  from  the 
passage  of  the  exhaust  steam  through  the  openings 
and  pipes.  Had  figure  29  been  taken  from  a  non-con- 
densing engine,  A  B  would  have  been  the  atmospheric 
line. 

Figure  30  we  have  copied  from  Main  and  Brown's 
Treatise  on  the  Indicator  and  Dynamometer.     It  was 


Fio.  30. 


taken  from  an  engine  fitted  with  the  long  D  slide. 
There  are  two  defects  exhibited  in  this  diagram ;  the 
steam  communication  is  opened  too  late  and  the  ex- 
haust too  soon.  At  C  the  exhaust  closes,  causing  the 
steam  to  be  compressed  to  O,  when  the  piston  having 
arrived  at  the  end  of  the'  stroke  starts  on  its  return, 
and  the  pressure  falls  to  O' ;  at  O'  steam  is  admitted, 
causing  the  line  O'  A  to  be  traced  ;  at  B  the  exhaust 
opens  long  before  the  piston  arrives  at  the  end  of  the 
stroke,  allowing  the  steam  to  escape  too  soon.  The 
hook,  as  shown  at  O,  would  only  be  made  in  very 
aggravated  cases,  where  the  steam  is  very  much  be- 
hind time. 


INDICATOR   DIAGRAMS. 


49 


Fig.  31  is  obtained  from  the  same  source  as  figure 
30.     In  this  case  the  engine  was  working  as  a  non- 

Fio.  81. 
B 


FIG.  32. 


condensing  engine  with  a  very  low  pressure  of  steam. 
The  exhaust  closes  at  A,  causing  the  pent-up  steam  to 
be  compressed  to  B,  where  the  steam  valve  opens, 
and  the  pressure  in  the  cylinder  being  greater  than 
that  in  the  boiler,  immediately  falls  to  C.  The  hook 
at  C  is  occasioned  by  the  momentum  of  the  indicator 
piston.  At  D  the  cut-off  closes,  causing  the  steam  to  be 
expanded  to  E,  below  the  atmosphere.  At  E  the 
exhaust  valve  opens  and  the  pressure  rises  up  equal  to 
the  back  pressure,  causing  the  loop  on  that  corner  of 
the  diagram. 

Figure  32  is  a  diagram  drawn  from  memory,  from 
one  of  a  non-condensing 
engine  that  was  once 
shown  the  author,  with 
the  request  that  he 
point  out  the  defect  in 
the  engine  from  which 
such  a  diagram  was 
taken.  At  first  we  did 
not  see  any  reason  why 
the  pressure  should  rise  from  b  to  <?,  for  supposing  the 
exhaust  to  open  at  3,  there  could  be  no  reason  why 
the  pressure  should  rise  beyond  ^/,  the  amount  of  back 
pressure  on  the  opposite  side  of  the  piston.  After 
looking  at  it  a  little  closer,  however,  it  occurred  to  us 


50 


INDICATOR    DIAGRAMS. 


that  such  a  diagram  could  be  formed  from  a  slide  valve 
engine,  and  in  this  manner :  Steam  being  admitted  in 
the  usual  way  until  the  piston  arrived  at  a,  the  inde- 
pendent slide  cut  off  the  steam,  whence  it  was  expanded 
to  the  point  b ;  at  I — the  steam  valve  having  neither 
lap  nor  lead,  and  consequently  still  open — the  cut-off 
again  opened  the  communication  with  the  cylinder, 
admitting  fresh  steam,  which  caused  the  line  I  c  to  be 
traced,  partaking  of  the  motion  of  the  steam  and  piston. 
At  c  the  steam  is  shut  off  by  the  steam  valve  itself, 
and  the  exhaust  opened,  the  pressure  therefore  falls 
from  that  point  to  ^/,  and  the  exhaust  line  is  traced. 
In  a  non-condensing  engine  diagram,  where  of  course 
there  can  be  no  vacuum  line,  the  line  from  c  to  e  in- 
clusive is  termed  the  exhaust. 

A  perfect  Diagram. — According  to  the  law  laid 
down  by  Marriotte,  which  we  have  previously  studied 
under  the  head  of  expansion  of  steam,  the  expansion 
curve  of  an  indicator  diagram  should  be  a  true  hyper- 


Fio.  33. 


bolic  curve,  were  there  no  extraneous  circumstances  to 
cause  it  to  be  otherwise  ;  but  unfortunately  in  practice 
this  perfection  is  not  attainable  were  Marriotte's  law 


INDICATOR   DIAGRAMS.  51 

literally  true,  owing  to  the  time  required  for  steam  to 
enter  and  leave  the  cylinder  clearance  of  piston,  space 
in  nozzles  between  the  valves,  leakage  of  valves,  piston 
condensation  in  the  cylinder,  <fec.  Fig.  33  is  intended 
to  show  a  perfect  diagram,  having  all  the  corners  well 
defined  and  the  expansion  line  a  true  hyperbolic  curve. 
From  this  figure  we  purpose  explaining  the  manner  of 
laying  out  a  true  hyberbolic  curve.  Let  A  E  be  the 
true  vacuum  line,  and  B  C  the  steam  line.  Divide 
A  E  into  any  number  of  equal  parts,  and  erect  the 
perpendiculars  A  B,  1 1',  2  2',  <fec.  Now  we  see  that 
the  steam  follows  the  distance  A,  2,  or  two  divisions 
before  it  is  cut  off  the  length  of  the  ordinate  3  3', 
being  three  divisions  from  the  commencement  should 
be  |  of  2,  0',  the  length  of  4  4',  f- ;  of  2,  C  ;  of  5  5'  f ; 
of  6  6'  |;  of  7  r  i ;  of  E  D,  |,  With  the  lengths 
of  all  these  ordinates  marked  on  the  diagram  drawn 
through  the  points  3',  4',  5',  6',  T',  <fec.,  the  line  C  I), 
and  you  have  the  required  curve. 

An  experienced  engineer  can  tell  at  a  glance 
whether  an  engine  is  in  good  working  order  from  its 
diagram ;  but  nevertheless,  in  most  cases  it  would  be 
well  to  draw  the  true  curve,  in  order  to  ascertain  how 
much  the  actual  one  differs  from  it,  for  by  this  means 
we  can  ascertain,  while  under  way,  whether  the  valves 
or  piston  leak;  but  in  drawing  the  true  curve,  the 
clearance  of  the  piston  and  space  in  the  nozzles,  &c., 
must  be  ascertained,  and  that  much  added  to  the  length 
of  the  diagram,  in  order  to  obtain  the  curve  accurately. 
Thus,  supposing  this  space  to  be  equal  in  capacity  to 
six  inches  in  length  of  the  cylinder,  make  the  diagram 
six  inches  longer  than  it  actually  is,  and  proceed  in  the 
manner  we  have  shown. 

Should  the  steam  valves  leak  while  every  thing 


5:2 


1 XDICATOR    DIAG  R  A  MS. 


else  remains  tight,  the  termination  of  the  expansion 
line  will  be  too  high,  and  if  the  exhaust  valves  or  pis- 
ton leak,  it  will  be  too  low, — allowance  beinor  made 

'  *  ~ 

for  condensation  in  the  cylinder. 


Steam 10  Ibs. 

Rev 9 

Vac 26 

Hot  well 100° 

Throttle  wide. 


FIG.  34. 

"  Powhatan  "  stb.  cylinder-top. 
February  13th,  1854. 


Figures  34  and  35  are  two  diagrams  taken  from 
the  U.  S.  Steamer  Powhatan,  on  the  13th  of  February, 
1854;  34  was  taken  about  ten  minutes  after  35.  In 
both  of  these  figures  we  have  the  true  hyperbolic 
curves  drawn  in,  with  and  without  taking  the  clear- 
ance, <fec.,  into  account.  The  upper  curve  in  small 
dots  is  the  true  curve,  when  the  clearance,  <fec.,  is 
taken  into  consideration,  and  the  lower  one  in  large 
dots  is  the  true  curve  without  reference  to  the  clear- 
ance, <fec.  In  figure  35,  where  the  steam  was  cut  off 
at  a  very  early  part  of  the  stroke,  the  importance  of 
taking  the  clearance,  <fec.,  into  consideration,  is  very 
conspicuous.  The  dotted  lines  on  the  right  of  these 


INDICATOR   DIAGKAMS. 


53 


diagrams  show  the  amount  they  are  lengthened  by 
adding  the  clearance,  space  in  nozzles,  <fec.,  to  them. 


Steam Si 

Rev 6 

Vac 26± 

Hot  well 82° 

Throttle  wide. 


FIG.  35. 

Ibs.  "  Powhatan"  stb.  cylinder-top. 

February  13th,  1854. 


From  a  casual  inspection  of  these  diagrams,  they 
seem  to  present  an  anomaly  that  at  first  is  difficult  to 
solve.  Thus,  in  figure  34,  the  termination  of  the  true 
expansion  curve,  considering  clearance,  <fec.,  is  about 
one  pound  above  the  actual  curve,  whereas  in  figure 
85  it  is  two  pounds  below  it.  The  first  would  indicate 
that  the  exhaust  valves  or  piston  leaked,  and  the 
second  that  the  steam  valves  leaked,  while  the  exhaust 
valves  and  piston  were  tight.  Now,  then,  since  one 
was  taken  only  about  ten  minutes  after  the  other,  it  is 
not  at  all  probable  that  this  sudden  change  was 
brought  about  in  that  short  space  of  time ;  hence  we 
must  look  for  some  defect  in  the  engine  that  would 
occasion  it.  We  account  for  it  in  this  way :  In  the 


54  INDICATOR    DIAGRAMS. 

first  case  the  steam  valve  leaked,  and  also  the  steam 
piston,  but  the  piston  leaked  to  a  greater  extent  than 
the  valve,  that  is  to  say,  more  steam  passed  through 
the  piston  and  into  the  condenser  from  the  leakage  of 
the  piston  than  entered  the  cylinder  from  the  leakage 
of  the  valve  ;  therefore,  the  actual  curve  must  fall  be- 
low the  true  curve.  In  the  second  case,  the  steam 
valve  also  leaked,  but  the  pressure  on  the  piston  fell 
so  rapidly,  from  expansion,  that  it  became  too  low  to 
force  a  passage  through  the  piston,  the  elasticity  of  the 
packing  being  sufficient,  in  this  case — though  not  in 
the  other,  where  it  had  a  greater  pressure  to  sustain- 
to  keep  it  tight ;  hence,  the  true  curve  in  this  case 
must  be  below  the  actual  curve,  agreeing  precisely 
with  the  conditions  of  the  figures. 

o 

There  is,  however,  another  thing  which  would  pro- 
duce diagrams  similar  to  those  before  us,  and  which 
most  probably  caused  the  formation  of  these,  viz.,  leak- 
age about  the  cylinder  heads.  Thus,  supposing  the 
stuffing  box,  for  instance,  to  leak.  So  long  as  the 
pressure  in  the  cylinder  remained  above  the  atmo- 
sphere, steam  would  blow  out,  occasioning  the  curve 
to  fall ;  on  the  other  hand,  when  cutting  off  short,  the 
pressure  in  the  cylinder  would  soon  fall  below  the 
atmosphere,  and  air  would  enter,  causing  the  curve  to 
rise,  exactly  as  shown  in  the  figures. 


INDICATOR    DIAGRAMS.  f>;> 

Fig.  36  is  a  diagram  taken  from  the  U.  S.  Steamer 
"  San  Jacinto,"  fitted  with  Allen  &  Wells'  cut-off. 

FIG.  30. 

Steam  in  boilers ll^lbs.  November  7th,  1855,  11J  A.  M. 

Revolutions 18  After  Engine,  inboard  end. 

Vacuum 25£  Coal  18  tons  in  24  hours. 

Hot  well 104° 

Throttle  4  holes  open.        Scale  =  Vio. 


From  inspection  of  the  expansion  curve  of  this  dia- 
gram, it  appears  that  this  cut-off  does  not  close  so 
quickly  as  Sickel's,  occasioning  the  corner  a  to  be 
more  rounding. 


J 


Steam  in  boilers  ......  9  Ibs. 

Revolutions  ............  5 

Hot  well  ................  100° 

Throttle....  4 


FIG.  37. 

"Powhatan,"  Feb.  13th,  1854. 
stb.  cylinder  bottom,  working  by  hand. 


Figure  37  is  a  diagram  showing  the  operation  of 


AIR-PUMP   DIAGRAMS. 


the  valves  while  working  by  hand.  This  valve  ex- 
hibits large  cushioning  arid  steam  lead,  the  exhaust 
valve  closing  at  «•,  and  the  steam  valve  opening  at  £, 
so  that  the  engine  actually  passed  the  centre  against  a 
pressure  of  6£  Ibs.  above  the  atmosphere. 


Steam 16.5  Ibs. 

Revolutions 0.25 

Hot  well 106° 

Vacuum  gauge  out  of  order. 


"  Powhatan  "  stb.  air-pump,  10.50  A.  M. 
January  18th,  1854. 


_    o 


Calculated  for  Vs  full  of  water. 

Number  1  is  a  diagram  taken  from  the  "  Powhat- 
an's  "  starboard  air-pump.  The  Powhatan's  air-pumps 
are  of  the  lifting  kind,  and  the  .piston  fitted  with  one 
large  brass  conical  valve.  We  will  explain  the  dia- 
gram. At  $,  the  piston  being  at  the  bottom  of  the 
stroke,  starts  to  rise,  compressing  the  air  and  vapor 
above  it,  until  it  arrives  at  #,  at  which  place  a  sudden 
discharge  of  air  and  vapor  seems  to  have  taken  place, 
and  the  pressure  fell  to  £,  from  which  point  the  pres- 
sure again  gradually  rose  until  it  arrived  at  <;/,  where 
the  water  began  to  be  delivered  and  continued  to  the 
end  of  the  stroke. 


AIR-PUMP    DIAGRAMS. 


.7 


Attached  to  the  top  of  the  air-pumps  is  a  pipe,  run- 
ning down  into  the  bilge,  for  the  purpose  of  pumping 
off  the  bilge  water.  Where  this  pipe  is  attached  to 
the  pump  is  fitted  a  valve,  operating  like  an  ordinary 
check  valve,  a  handle  being  made  to  screw  down  on 
the  top  of  it  to  keep  it  firmly  in  its  seat,  when  there  is 
no  water  in  the  bilge. 


Steam lolbs.    "Powhatan."  stb.  air-pump,  10.55  A.  M. 

Revolutions. 10  January  18th,  1854. 

'  Hot  well, 106° 

Vacuum  gauge  out  of  order. 


Resistance  of  vapor  and  water  in  Air-pump 


=  (6.6-j-  312  x  .0969=)  .6697  lb.  per  square  inch  of  steam  piston. 
Calculated  for  '  3  full  of  water. 

There  being  no  water  in  the  bilge  at  the  time  ~No.  1 
was  taken,  No.  2  was  taken  five  minutes  after,  for  the 
purpose  of  ascertaining  what  effect  the  opening  of  this 
valve  and  admitting  air  would  produce.  It  shows 
that  no  extra  power,  from  the  admission  of  this  air, 
was  required  to  work  the  pump,  the  average  pressure 
being  about  the  same  as  in  No.  1,  and  that  the  vacuum 
in  the  pumps  at  no  time  was  more  than  4^  Ibs.  There 
was  no  alteration  in  the  vacuum,  as  shown  by  the 
gauge,  however  attached  to  the  condenser,  and  the 
engines  continued  to  work  in  the  same  manner  as  be- 


58 


AIll-PUMP   DIAGRAMS. 


fore  the   bilge  valve    attached   to  the  air-pump  was 
opened. 

Steam 15lbs.  " Powhatan  "  port  air-pump,  11.6A.M. 

Revolutions 10  January  18th,  1854. 

Hot  well 108° 

Vacuum  gauge  out  of  order. 


Resistance  of  vapor  and  water  in  Air-pump 

=  (0.46 -f. 223  x  .0969  =)  .6476  Ib.  per  square  inch  of  steam  piston 
Calculated  for  '/T  full  of  water. 

Nos.  3  and  4  were  taken  in  the  same  manner  from 
the  port  air-pump  a  few  minutes  after  1  and  2  were 
taken  from  the  starboard  pump. 

In  these  diagrams  the  pressure  at  the  termination 
of  the  up  stroke,  it  will  be  seen,  is  about  2^  Ibs.  per 
square  inch  above  the  atmosphere,  which  is  due  to  the 
height  of  the  level  of  the  water  surrounding  the  ship 
above  the  top  of  the  air-pump.  The  pressure  increased 
to  between  7  and  8  Ibs.  per  square  inch,  as  shown  in 
other  parts  of  the  diagram,  is  occasioned  by  the  fric- 
tion of  the  water  and  vapor  through  the  delivery  pipes 
and  valves.  The  slanting  off  in  the  diagram,  'No.  1, 
from  x  to  y,  we  think  partly  owing  to  two  causes : 
First,  the  decreased  velocity  of  the  piston  as  it  ap- 


AIK-PUMP   DIAGEAMS.  &V 

preaches  the  end  of  its  stroke  does  not  expel  the  water 
with  such  force,  and  hence  there  is  not  so  much  fric- 

Steam 15lbs.     "  Powhatan  "  port  air-pump,  11.15A.M. 

Revolutions 10  January  8th,  1854. 

Hot  well 108° 

Vacuum  gauge  out  of  order. 


Resistance  of  vapor  and  water  in  air-pump 

=  (8.16 -(-'223  x  .0969=)  .8123  Ibs.  per  square  inch  of  steam  piston. 
Calculated  for  J/7  full  of  water. 

tion ;  but  this  would  not  occasion  the  slanting  off  from 
h  to  z  on  the  return  stroke ;  and  secondly,  there- 
fore, we  are  inclined  to  think  that  the  string  slipped  or 
stretched  a  little  from  a?  to  y,  and  recoiled  again  to  its 
original  place  from  li  to  z. 

We  will  npw  proceed  to  ascertain  the 

Power  required  to  work  the  Air-puny). 

Ascertain  the  capacities  of  the  steam  cylinder  and 
air-pump,  by  multiplying  the  areas  of  their  cross-sections 
by  the  lengths  of  their  strokes,  and  divide  the  latter 
by  the  former,  which  will  give  the  r atio  of  the  cylinder 
capacity  to  that  of  the  air-pump.  But  the  air-pump 
makes  but  one  delivery  stroke  to  every  two  strokes  of 
the  steam  piston,  consequently  divide  this  ratio  by 
two,  which  Avill  give  the  coefficient  for  our  present 


60          POWER    REQUIRED    TO    WORK    THE    AIR-PUMP. 

calculation,  and  this  coefficient  multiplied  by  the  mean 
pressure  per  square  inch  of  air-pump  piston — which 
can  be  ascertained  from  an  indicator  diagram — will 
give  the  mean  pressure  per  square  inch  required  to 
expel  the  air  and  vapor. 

This  of  course  must  be  augmented  by  the  weight 
of  the  water  raised. 

The  indicator  diagram  will  show  very  nearly  at 
what  part  of  the  stroke  the  pump  begins  to  deliver 
the  water,  and  therefore  what  fraction  of  the  pump  is 
filled,  from  which  can  be  easily  ascertained  the  number 
of  cubic  feet  of  water  lifted ;  and  this  number  multi- 
plied by  64.3  or  62.5,  as  the  vessel  may  be  running  in 
salt  or  fresh  water,  will  give  the  number  of  pounds. 
And  the  number  of  pounds  of  water  lifted,  divided  by 
the  area  of  the  air-pump  piston,  and  multiplied  by  the 
coefficient  before  obtained,  will  give  the  pressure  per 
square  inch  of  steam  piston  required  f  o  expel  the  water 
from  the  pump. 

The  sum  of  these  results  will  give  the  pressure  per 
square  inch  of  steam  piston  required  to  work  the  air- 
pump  independent  of  friction,  an  amount  that  is  usually 
estimated. 

Example:  The  capacity  of  the  "Powhatan's"  cyl- 
inder, i.  £.,  the  space  displaced  by  the  steam  piston  per 
stroke,  is  267.25  cubic  feet:  ditto  in  air-pump,  51.8 
cubic  feet ;  proportion  of  steam  piston  displacement  to 
that  of  half  of  air-pump  piston  displacement,  1.000  to 
.0969;  area  of  air-pump  piston,  2134  square  inches. 
The  pump  was  filled  \  full  of  water,  as  shown  by  dia- 
gram No.  1,  and  the  mean  pressure  throughout  the 
stroke  was  6.5  Ibs.  per  square  inch ;  hence,  6.5  X  .0969 
=  .6298  Ib.  per  square  inch  of  steam  piston  resistance 
from  vapor  in  air-pump,  and  .312  x  .0969  —  .0302  Ib. 


POWER    REQUIRED    TO    WORK   THE    AIR-PUMP.          61 

per  square  inch  of  steam  piston  resistance  from  the 
weight  of  water  lifted ;  total  =  (.6298  -f  .0302  =)  .66 
pounds  per  square  inch  of  steam  piston,  required  to 
work  the  air-pump,  independent  of  friction. 

Now,  supposing  the  mean  unbalanced  pressure  on 
the  steam  piston  per  square  inch  to  have  been  20  Ibs., 
we  have  20  :  .66  :  :  100  :  3.3  per  cent,  of  the  total 
power  of  the  engine  required  to  work  the  air-pump. 


CHAPTER  III. 


FIG.  42 


THE    HYDROMETER. 

THE  Hydrometer  is  an  instrument  used  for  the 
purpose  of  determining  the  specific  gravi- 
ties of  liquids.  When  applied  to  the 
water  of  marine  boilers,  it  indicates  the 
amount  of  saline  matter  the  water  con- 
tains. Figure  42  shows  the.  kind  of  hy- 
drometer usually  used  on  board  Ameri- 
can steamers.  The  lower  globe  is  filled 
with  shot,  or  other  weighty  substance, 
for  the  purpose  of  keeping  the  instru- 
ment upright.  When  the  hydrometer  is 
placed  in  fresh  water,  the  point  O  stands 
even  with  the  surface  of  the  water ;  when 
placed  in  water  containing  one  pound  of 
saline  matter  in  thirty-two  pounds  of 
water,  it  stands  at  %2 ;  when  the  water 
contains  two  pounds  of  saline  matter  in 
thirty  two  pounds  of  water  it  stands  at 
%2,  and  so  on.  So  that  by  placing  this 
instrument  in  a  small  quantity  of  water, 
drawn  from  the  boilers  at  intervals,  it 
will  show  the  exact  density,  by  which 
we  know  how  to  regulate  the  bio  wing-off. 

In  the  boilers  of  sea-going  vessels  the 
water  is  usually  carried  from  1%  to  "2  per 
hydrometer,  i.  <?.,  from  the  point  a  to  /;, 
figure  42.  In  the  Gulf  of  Mexico,  how- 
ever, in  the  vicinity  of  the  Florida  reefs,  where  the 


«*/ 
32 


THE   HYDROMETER.  63 

water  is  impregnated  with,  an  unusual  amount  of  lime, 
it  is  found  not  to  be  prudent  to  carry  it  beyond  iy2. 

The  hydrometer,  when  made  for  a  certain  temper- 
ature, is  not  adapted  to  any  other,  but  the  water 
should  be  allowed  to  cool  down  to  the  temperature 
marked  on  the  hydrometer  before  observing  the  indi- 
cation, and  for  this  purpose  it  becomes  necessary  also 
to  use  a  thermometer.  The  hydrometers  used  in  this 
country  are  usually  graduated  for  a  temperature  of 
200°  Fahr.  We  can  allow,  however,  for  a  few  degrees 
either  above  or  below  this  figure,  without  appreciable 
error — a  difference  of  10°  in  temperature  making  a 
difference  of  about  an  eighth  of  %2  in  the  scale.  Thus, 
supposing  the  water  to  be  at  a  temperature  of  210°, 
and  the  hydrometer  graduated  for  200°  to  stand  at  #, 
or  1%,  the  actual  density  of  the  water  will  not  be  1%, 
but  !7/8,  or  halfway  between  a  and  1).  On  the  other 
hand,  if  the  temperature  be  190°,  and  the  hydrometer 
stand  at  1%,  the  true  density  will  be  1%.  Neverthe- 
less, in  practice,  it  is  always  best  to  allow  the  water  to 
cool  to  the  temperature  for  which  the  hydrometer  is 
graduated,  whenever  it  can  be  done  without  the  waste 
of  too  much  time. 

It  will  be  observed  that  the  divisions  on  the  scale 
are  not  of  equal  lengths.  Thus :  the  distance  from  O 
to  %2  ig  greater  than  from  %2  to  %2,  and  from  %2  to 
%2j  greater  than  from  %2  to  %2,  and  so  on.  The  reason 
of  this  can  be  explained  in  this  manner :  When  the 
instrument  stands  at  O,  the  two  bulbs,  and  all  the 
tube  below  O,  of  course,  are  immersed,  having  the 
weight  due  to  the  length  of  the  tube  only  above  O  to 
support.  When  it  rises  to  y32  it  has  more  weight  to 
support,  from  the  fact  of  there  being  more  tube  out 

of  water,  and  it  also  has  less  bulk  immersed  ;  at  %2  it 
5 


64  LOSS   BY   BLOWING   OFF. 

has  still  more  weight  to  support,  while  there  is  still 
less  of  the  instrument  immersed,  and  so  on  down  to 
the  bottom  of  the  scale,  occasioning  the  lengths  of  the 
divisions  to  become  less  and  less. 

The  proportional  quantity  of  saline  matter  con- 
tained in  sea  water,  at  different  localities,  varies  very 
considerably,  as  will  be  seen  in  the  following 


TABLE : 


Baltic  Sea,  . 
Black  Sea, 
Arctic  Sea, 
Irish  Sea,        . 
British  Channel, 


Mediterranean,  i 

Atlantic  at  Equator.        .         .     as 

South  Atlantic,    . 

North  Atlantic, 

Dead  Sea,    .... 


LOSS    BY   BLOWING    OFF. 


When  water  contains  3  per  cent,  by  weight  of  sa- 
line matter,  no  deposit  takes  place  at  the  boiling 
point ; — under  atmospheric  pressure  or  212°  Fahr. 
When  it  contains  10  per  cent,  it  makes  a  deposit  of 
lime,  principally  sulphate,  and  at  29,  5  per  cent,  com- 
mon salt. 

The  precise  saturation,  however,  at  which  deposit 
commences  to  take  place  is  not  well  established,  but 
there  is  one  thing  which  is  well  known,  and  that  is, 
the  higher  the  temperature  of  the  water,  the  greater 
will  be  the  deposit,  and  from  this  we  conclude  that 
common  sea  water  would  deposit  a  portion  of  its  saline 
matter  if  heated  to  a  sufficiently  high  temperature. 
The  reason  of  the  increase  in  deposit,  as  the  tempera- 
ture is  increased,  is  probably  owing  to  the  expansion 
of  the  water,  or  the  separation,  as  it  were,  of  the  par- 
ticles. 

Water  carried  at  a  density  that  would  cause  no 


LOSS    BY    BLOWING    OFF.  65 

deposit  at  a  temperature  of  220°,  would  make  consid- 
erable deposit  at  a  temperature  of  260°  or  270°;  and 
this  is  the  reason  why  we  are  limited  to  comparatively 
low  steam  in  boilers  using  sea  water.  Independent  of 
the  saving  of  the  loss  by  blowing  off,  repairs  to  boilers, 
labor  of  cleaning  them,  <fec.,  this  is  a  powerful  reason  why 
inventive  genius  should  endeavor  to  bring  forth  a  relia- 
ble fresh  water  condenser,  and  why  steamship  owners 
and  others,  having  it  within  their  power,  should  encour- 
age all  such  attempts,  from  the  fact  of  the  great  advan- 
tage to  be  derived  from  carrying  high  pressure  steam, 
and  using  the  expansive  principle  to  its  fullest  extent. 

To  the  minds  of  those  who  cannot  clearly  see  that 
an  increase  of  temperature  occasions  an  increase  in  de- 
posit, a  practical  demonstration  can  be  obtained  by 
examining  the  crown  sheets,  and  other  parts  of  marine 
boilers,  subject  to  the  highest  temperature,  where  it 
will  be  found  the  largest  deposit  takes  place. 

The  deposit  of  lime,  or  "  scale,"  as  it  is  technically 
termed,  on  the  heating  surface  of  boilers,  being  nearly  a 
non-conductor  of  caloric,  prevents  a  large  portion  of  the 
heat  from  entering  the  water,  allowing  it  to  escape  up 
the  chimney,  and  is  therefore  lost ;  and,  if  the  deposit 
of  scale  be  large,  the  metal  of  the  boiler,  being  no 
longer  protected  by  the  water,  becomes  over-heated 
and  "  burnt."  To  prevent  these  results,  a  portion  of 
the  water  is  extracted  periodically,  or  continuously, 
by  the  brine  pump,  or  is  discharged  by  the  blow-off, 
in  order  to  keep  the  density  of  the  water  below  the 
point  at  which  any  serious  deposit  may  take  place. 
But  as  all  the  water  discharged  from  the  boiler  has 
first  to  be  heated,  and  as  it  is  replaced  by  water  of  a 
lower  temperature,  a  loss  of  heat  (which  is  virtually  a 
loss  of  fuel)  is  occasioned  thereby.  This  is  technically 


66  LOSS    BY   BLOWING    OFF. 

termed  "  loss  by  blowing  off,"  and  we  shall  proceed  to 
illustrate  the  manner  of  calculating  it.  Take  an  ex- 
ample. 

Supposing  the  density  of  the  water  entering  the 
boiler  to  be  -gV,  and  that  of  the  boiler  to  be  maintained 
at  -g2¥,  there  will  be  one  part  converted  into  steam,  and 
one  part  blown  out.  Supposing  also  the  temperature 
of  the  water  entering  the  boiler  to  be  100°  Fahr.,  and 
the  temperature  of  the  water  in  the  boiler  to  be  248° 
Fahr.,  we  have  all  the  data  required. 

Referring  to  Regnault's  experiments,  (page  9),  we 
see  that  the  total  heat  in  steam  having  248°  for  the 
sensible  heat,  is  1189.58°,  now  then 

1189.58°  =  total  heat; 
100.00°  =  temperature  of  the  water  entering  the 

boiler ; 
1089.58°  =  heat  required  from  the  fuel  for  the 

water  to  be  evaporated ; 
248°  =  temperature  of  the  water  in  the  boiler ; 
1000=          "  "  "     entering     " 


148°  =  heat  lost  by  blowing  off. 

Therefore,  since  one  part  (requiring  1089.58°)  is 
converted  into  steam,  and  the  other  part  (requiring 
148°)  is  blown  off,  the  total  heat  required  of  the  fuel  is 
(1089.58°  +  148°  =)  1237.58°;  and  as  148°  of  this  is 
blown  off,  we  have  1237.58  :  148  :  :  100  :  11.95  +  per 
cent,  loss  by  blowing  at  the  above  density  and  tem- 
perature. 

If  the  water  had  been  carried  at  a  density  of  If 
per  hydrometer,  one  part  would  have  been  blown  off 
as  before,  but  only  three-quarters  part  would  have 


LOSS    BY   BLOWING    OFF.  67 

been  converted  into  steam,  hence  we  would  have  pro- 
ceeded thus — 

1189.58° 
100.00° 


1089.58° 
.75° 

817.1850°  =  heat  required  from  the  fuel  for  the 
water  to  be  evaporated. 

248° 
100° 


148°  =  heat  lost  by  blowing  off. 

Therefore    (817.185  +  148  =)  965.185  :  148  : :  100  : 
15.33  +  per  cent, 

And  had  the  water  been  carried  at  a  density  of 
3,  i.  e.  -fa,  two  parts  would  have  been  used  for  steam 
and  one  part  blown  off,  hence  the  following : 

1189.58° 
100.00° 


1089.58° 

2° 

2179.16°  =  heat  required  from  the  fuel  for  the 
water  to  be  evaporated. 

248° 
100° 


148°  =  heat  lost  by  blowing  off. 

Therefore  (2179.16°  +  148°)  =  2327.16°  :  148  :  :  100 : 
6.35  per  cent.,  and  so  on  for  any  density.     These  per 


08  GAIN    BY    THE    USE   OF   HEATERS. 

cents,  are  the  losses  in  fuel,  combustible,  minus  that 
lost  from  radiation  and  heated  gases  passing  up  the 
chimney. 

The  above  calculations  apply  only  to  cases  where 
the  water  enters  the  boiler  at  a  density  of  ^ ;  should 
it  enter  at  a  lower  density,  the  loss  will  be  less,  or  a 
greater  density  more,  because  to  retain  the  water  in 
the  boiler  at  the  density  assumed  in  the  above  ex- 
amples, there  would  either  have  to  be  a  less  or  greater 
quantity  blown  off  than  we  have  considered  to  be  the 
case. 

In  order  not  to  lose  entirely  all  the  heat  in  the 
water  blown  off,  some  boilers  are  fitted  with  heaters, 
or  as  they  are  sometimes  termed  incorrectly,  "  refrige- 
rators." These  are  a  series  of  pipes  surrounded  by  the 
feed  water,  and  through  which  the  water  leaving  the 
boilers  has  to  pass ;  by  this  means  the  temperature  of 
the  feed  water  is  considerably  increased  before  it 
enters  the  boiler.  The  following  will  illustrate 


THE  GAIN  BY  PUMPING  WATER  INTO  THE  BOILER  AT  AN 
INCREASED    TEMPERATURE. 

For  this  purpose  two  examples  will  be  sufficient, 
and  we  will  commence  with  the  first  one  given  above 
in  the  calculation  on  the  loss  by  blowing  off;  viz. 
steam,  248° ;  feed  water,  100°  ;  and  density,  -^.  Now 
suppose  by  the  application  of  the  heater,  the  feed- 
water,  instead  of  entering  the  boiler  at  100°,  is  made 
to  enter  at  150°,  what  will  be  the  saving  in  fuel  by  its 
application  ? 


GAIN    BY   THE    USE    OF    HEATEES.  69 

Solution. 

1189.58°  —  total  heat  in  the  steam ; 
100.00°  =  temperature  of  the  feed  water ; 


1089.58°  =  heat  required  from  the  fuel  to  evapo- 
rate one  part  of  water  ; 
248°  =  temperature  of  the  water  blown  off; 
100°=         "  "          feed  water; 


148°  =  heat  lost  by  blowing  off; 
and  1089.58°  +  148°  =  1237.58  =  total  heat  required 
from  the  fuel  where  the  water  is  pumped  into  the 
boiler  at  100°.  Let  us  now  see  what  the  total  heat 
will  be  when  the  water  is  pumped  in  at  150°,  and  the 
difference  between  these  results  will  be,  of  course,  the 
saving 

1189.58°  =  total  heat  in  the  steam ; 
150.00°  =  temperature  of  the  feed  water ; 

1039.58°  =  heat  required  from  the  fuel  to  evapo- 
rate one  part  of  water  ; 
248°  =  temperature  of  the  water  blown  off ; 
150°=         "  "          feed  water; 


98°  =  heat  lost  by  blowing  off ; 

and  1039.58°  +  98°  =  1137.58°  =  total  heat  required 
from  the  fuel  when  the  water  is  pumped  into  the  boiler 
at  150°.  Therefore 

1237.58° 
1137.58° 


100°=:  saving  in  degrees; 

whence  1237.58°  :  100° : :  100  :  8.08  per  cent.     That  is 
to  say,  if  without  the  heater  the  boilers  consumed  100 


TO  GAIN    BY    THE    USE    OF   HEATEKS. 

tons  of  coal  per  day,  with  it  they  would  produce  the 
same  quantity  of  steam  with  91.92  tons. 

EXAMPLE  2D. 

Suppose  that  the  density  of  the  water  in  Example  1 
was  If,  and  all  the  other  conditions  to  remain  unalter- 
ed, what  would  be  the  saving  in  that  case  ? 

Solution. 

1189.58°  =  total  heat  in  the  steam ; 
100.00°  =  temperature  of  the  feed  water ; 

1089.58°  =  heat  required  from  the  fuel  to  evapo- 
rate one  part  of  water  ; 
.75°  =  part  of  water  evaporated  ; 


817.185°  •=  heat  required  from  the  fuel  for  the 

water  that  is  evaporated  ; 
248°  —  temperature  of  the  water  blown  off ; 
100°=         "  "          feed  water; 


148°  =  heat  lost  by  blowing  off; 
817.185°+  148°  =  965.185°  =  total  heat  required  from 
the  fuel  when  the  water  is  pumped  into  the  boiler  at 
100°. 

1189.58°  =  total  heat  in  the  steam  •, 
150.00°  =  temperature  of  tho  feed  vrr.ter; 

1039.58°  =  heat  required  from  the  fuel  to  evapo- 
rate one  part  of  water ; 
.75°  =  part  of  water  evaporated; 

779.685°  =  heat  required  from  the  fuel  for  the 
water  that  is  evaporated ; 


INJECTION    WATEE.  71 

248°  =  temperature  of  the  water  blown  off; 
150°=         "  "  feed  water; 


98°  =  heat  lost  by  blowing  off; 

779.685°  +  98°  =  877.685°  =  total  heat  required  from 
the  fuel  when  the  water  is  pumped  into  the  boiler  at 
150°.  Therefore 

965.185° 

877.685° 


87.5°=  saving  in  degrees. 

Whence  965.185°  :  87.5° : :  100  :  9.06  per  cent.  And 
in  this  manner  the  calculation  can  be  made  for  any 
density  and  temperature. 

In  making  calculations  on  the  theoretical  saving 
from  the  use  of  the  heater,  we  have  seen  some  engineers 
who  calculate  the  loss  by  blowing  off  without  it,  and 
again  with  it,  and  take  the  difference  between  these 
two  results  for  the  saving;  but  it  will  require  but 
little  reflection  for  any  one  at  all  conversant  with  such 
subjects,  to  perceive  the  .error  of  this  mode  of  calcu- 
lation, as  it  takes  no  cognizance  whatever  of  the  extra 
heat  given  to  that  portion  of  the  water  which  is  evap- 
orated. The  mode  of  calculation  given  above  is  the 
only  correct  one,  as  it  takes  into  consideration  all  the 
elements. 

INJECTION  WATEE. 

After  the  steam  has  performed  its  duty  in  the 
cylinder,  and  been  exhausted  into  the  condenser,  a  cer- 
tain amount  of  cold  water  is  admitted  into  that  vessel 
for  the  purpose  of  condensing  it,  and  this  quantity 
depends  upon  the  temperatures  of  the  water  and  the 
steam.  We  will  take  an  example. 


72  EVAPORATION. 

Suppose  the  temperature  of  the  injection  water  to 
be  60°;  steam  as  it  enters  the  condenser,  212°;  and 
water  in  the  condenser,  110°.  Required  the  proportion 
of  injection  water  to  the  water  evaporated  in  the 
boiler : 

Solution. 

1 1*78.6°  =  total  heat  in  the  steam  at  the  sensible 

temperature  of  212° ; 

110.0°  =  temperature  of  the  water  after  conden- 
sation ; 
1068.6°  =  heat  to  be  destroyed ; 

110°  =  temperature  of  the  water  after  conden- 
sation ; 
60°  =  temperature  of  the  injection  water, 


50°  difference. 

Now  then  we  see  that  we  have  1068.6°  of  heat  to 
be  destroyed,  and  only  50°  to  do  it  with,  therefore  we 
must  make  up  this  difference  in  quantity ;  hence  1068.6° 
-f-50°  =  21.372  times  the  evaporated  water  to  be  ad- 
mitted into  the  condenser  to  condense  the  steam  and 
retain  the  condenser  at  the  temperature  of  110°. 

EVAPORATION. 

Among  the  important  elements  to  be  ascertained 
in  the  performance  of  the  steam  engine,  is  the  quantity 
of  water  evaporated  in  the  boilers  per  unit  of  coal,  or 
other  fuel.  In  sea  boilers  using  salt  water,  one  pound 
of  coal  evaporates  from  4  to  9  pounds  of  water,  de- 
pendent upon  the  quality  of  the  coal,  the  construction 
and  cleanliness  of  the  boilers.  Those  boilers  are  of 
course  the  best  which  evaporate  the  largest  quantity, 
and  hence  the  importance  of  knowing  the  exact  per- 
formance of  each  boiler,  as  well  as  of  the  different  kinds 


EVAPORATION.  73 

of  fuels  used  in  the  same.     To  secure  this  desirable  end 
we  proceed  thus : 

Ascertain  from  indicator  diagrams  the  fraction  of 
the  cylinder  filled  at  each  stroke,  from  which,  know- 
ing the  diameter  of  the  cylinder,  we  ascertain  the 
number  of  cubic  feet  of  steam  required  to  fill  that 
space,  and  to  this  we  add  the  space  in  nozzles,  clear- 
ances, &c.,  which  gives  the  number  of  cubic  feet  of 
steam  used  per  stroke ;  and  the  number  of  cubic  feet  of 
steam  used  per  stroke,  multiplied  into  the  number  of 
strokes  per  hour,  and  divided  by  the  relative  volumes 
of  steam  and  water,  at  the  pressure  the  steam  is  admit- 
ted into  the  cylinder,  gives  the  number  of  cubic  feet 
of  water  evaporated  per  hour,  and  the  number  of  cubic- 
feet  of  water  evaporated  per  hour,  multiplied  by  64.3, 
(the  weight  in  pounds  avoirdupois  of  one  cubic  foot  of 
sea  water,)  and  divided  by  the  number  of  pounds  of 
coal  used  per  hour,  gives  the  number  of  pounds  of  water 
evaporated  per  pound  of  coal,  provided  there  is  no 
blowing  off  done ;  but  wherever  there  is  blowing  off, 
this  last  result  has  to  be  increased  to  the  extent  of  the 
loss  by  blowing. 

Suppose  for  instance,  proceeding  in  the  manner 
given  above,  we  find  6  Ibs.  of  water  to  be  evaporated 
per  pound  of  coal ;  and  the  loss  by  blowing  off  to  keep 
the  water  at  the  proper  density  to  be  15  per  cent., 
the  remaining  85  per  cent,  is  that  which  evaporates 
the  6  Ibs. ;  hence  85  :  6  : :  100  :  7.06  Ibs.  of  water  evap- 
orated per  pound  of  coal. 

EXAMPLE. — Suppose  you  have  a  cylinder  70  inches 
diameter  by  10  feet  stroke ;  the  initial  pressure  of 
steam  in  the  cylinder  24.5  Ibs.  per  square  inch,  in- 
clusive of  the  atmosphere,  cut  off  at  \  from  commence- 
ment of  stroke ;  clearance,  <fec.,  10  cubic  feet ;  revolu- 


74  EVAPOEATION. 

tions,  15  per  minute;  coal  consumed  per  hour,  1,500 
Ibs.;  water  carried  at  If  per  hydrometer;  temperature 
of  feed  water,  107°  Fahr.  ;  required  the  number  of 
pounds  of  water  evaporated  per  pound  of  coal  : 

Solution. 
W         854  X  ~  +  10  =  76.8125    cubic    feet    of 


144  4 

steam  used  per  stroke;  and  76.8125  x  15  x  2  X  60  — 
138262.5  cubic  feet  of  steam  used  per  hour. 

The  relative  volumes  of  steam  and  water  at  the 
pressure  of  24.5  Ibs.  are  1064  to  1  ;  hence 

1S8262  5 

rj—  X  64.3  —  1500  =  5.57  Ibs.  of  water  per  pound 
1064 

of  coal,  neglecting  the  loss  by  blowing  off;  but,  ac- 
cording to  the  conditions  of  the  example,  the  loss  by 
blowing  off  is  found  to  be  14.1  per  cent.,  the  remain- 
ing 85.9  per  cent,  is  that  therefore  which  evaporated 
the  5.57  Ibs.  of  water;  hence  the  true  evaporation  is 
found  to  be  85.9  :  5.57  :  :  100  :  6.48  Ibs.  of  water  per 
pound  of  coal. 

The  above  calculation  takes  no  cognizance  of  the 
leakage  of  the  valves,  loss  by  radiation,  or  condensa- 
tion in  the  cylinder,  pipes,  <fec.  ;  hence  the  results  show 
too  small,  but  it  is  the  only  standard  of  comparison. 

Some  parties  calculate  the  evaporative  power  of 
boilers  by  measuring  the  quantity  of  water  pumped 
into  them  during  any  given  time,  and  also  the  quantity 
of  coal  consumed  in  the  furnaces  during  the  same  time, 
and  dividing  the  weight  of  the  former  by  the  latter, 
which  they  conceive  gives  the  weight  of  water  evapo- 
rated per  unit  of  coal.  Upon  first  sight  this  mode  of 
operating  appears  very  simple  and  correct  ;  but  unfor- 
tunately, notwithstanding  its  simplicity,  the  results  are 


STEAM  AND  VACUUM  GAUGES.  75 

never  accurate,  the  evaporation  being  always  shown 
too  large,  for  the  very  simple  reason,  that  all  the  water 
pumped  into  a  steam  boiler  is  never  evaporated.  All 
boilers,  and  pipes,  and  cocks  attached  thereto,  leak 
more  or  less,  and  sometimes  boilers  foam,  occasioning 
water  to  be  worked  into  the  cylinders,  and  as,  accord- 
ing to  this  mode  of  calculation,  all  water  escaping  by 
this  means  is  supposed  to  be  evaporated,  the  result 
manifestly  cannot  be  correct. 

Steam  and  vacuum  Gauges. 

As  applied  to  the  marine  steam  engine,  the  mer- 
curial steam  and  vacuum  gauges  are  the  most  common, 
though  of  late  years  there  have  come  into  use  a  variety 
of  metallic  gauges,  many  of  which,  from  the  little 
attention  they  require,  appear  to  be  very  well  adapted 
to  the  purpose  for  which  they  were  intended. 

The  most  prominent  of  these  are  "  Schaffer's," 
"Hearson's,"  "Schmidt's,"  " Ashcroft's,"  "Eastman's," 
" Stubblefield's,"  and  "Allen's."  In  the  first  three, 
the  spring  is  a  thin  corrugated  plate,  upon  which  the 
steam  acts,  communicating  motion  to  a  hand  or  pointer 
which  moves  around  a  circular  disc  marked  in  pounds : 
the  spring  in  Ashcroft's  gauge  is  a  bent  tube,  which 
the  elasticity  of  the  steam  tends  to  straighten.  East- 
man's gauge  is  a  combination  of  springs  and  levers. 
As  these  gauges  are  all  constructed  on  the  same  prin- 
ciple, viz.,  the  elasticity  of  metal,  we  shall  not  stop 
here  to  describe  them,  as  it  is  more  directly  our  object 
to  deal  with  principles,  rather  than  mechanical  ar- 
rangements, which  are  the  chief  peculiarities  of  these 
gauges.  We  will  pass  on  to  the  mercurial  closed  top 
vacuum  gauge. 


STEAM   AND   VACUUM   GAUGES. 


Fig.  43. 


26 


a  b  c  d,  figure  43,  is  a  basin  filled 
with  mercury  up  to  the  point  A  ;  the 
tube  B  is  also  filled  with  mercury. 
The  pipe  e  communicates  with  the 
condenser,  and  when  that  vessel  is 
filled  with  air  of  the  atmospheric 
pressure,  the  surface  of  the  mercury 
in  the  basin  is  pressed  with  a  pres- 
sure of  about  15  Ibs.  per  square  inch, 
causing  the  tube  B  to  remain  filled ; 
but  when  a  partial  vacuum  is  created 
in  the  condenser,  the  mercury  having 
no  longer  the  atmospheric  pressure 
to  sustain,  falls  in  the  tube  B,  and 
the  figures  marked  on  the  scale  will 
exhibit  the  extent  of  the  vacuum. 
With  this  arrangement,  therefore,  there  is  no  necessity 
of  making  the  tube  30  inches  in  length,  as  all  engines 
are  supposed  to  maintain  at  least  17  or  18  inches  of 
vacuum,  and  a  tube  long  enough  to  show  this  is  all 
that  is  required.  Could  the  surface  of  the  mercury 
remain  constantly  at  A,  the  divisions  on  the  scale 
would  be  of  equal  lengths,  and  one  inch  apart,  but  as 
the  mercury  rises  a  little  in  the  reservoir  as  it  falls  in  the 
tube,  the  lengths  of  these  divisions  vary  a  little,  depend- 
ent upon  the  relative  volumes  of  the  tube  and  reservoir. 
The  aperture  in  the  lower  end  of  the  tube  is  made 
very  small,  to  prevent  the  oscillation  of  the  mercury. 
At  A  is  a  small  hole  fitted  with  a  screw.  This  is  left 
open,  while  filling  the  gauge,  as  an  overflow  to  the 
surplus  mercury,  it  being  so  situated  that  the  contents 
of  the  tube  B  is  just  sufficient  to  fill  the  reservoir  to 
the  point  30,  or  the  true  vacuum  line. 

The  pressure  of  the  atmosphere,  as  it  varies  from 
time  to  time,  does  not  alter  the  indications  of  this 


STEAM   AJSD    VACUUM    GAUGES. 


77 


FIG.  44. 


gauge,  inasmuch  as  it  always  exhibits  the  difference 
between  the  vacuum  in  the  condenser  and  a  perfect 
vacuum. 

Had  the  top  of  the  tube  B  communicated  with  the 
condenser,  and  the  basin  abed  been  open  to  the 
atmosphere,  the  gauge  would  then  have  been  what  is 
termed  an  open-top  vacuum  gauge,  and  would  require 
to  have  been  30  inches  in  length — the  scale  being 
reversed,  the  lowest  figure  commencing  at  the  bottom. 
With  such  a  gauge,  all  variation  in  the  pressure  of  the 
atmosphere  affects  its  indications. 

Figure  44  is  a  siphon  steam  gauge, 
filled  with  mercury  to  the  level  a  a. 
The  short  leg  connects  to  the  boiler, 
and  the  long  leg  is  open  to  the  atmo- 
sphere. The  steam  pressing  upon  the 
mercury  at  «,  forces  up  the  stick  resting 
on  the  mercury  in  the  other  leg  at  «', 
showing  the  pressure  in  pounds  per 
square  inch,  marked  on  the  scale  at  the 
top  of  the  gauge.  These  divisions  are 
one  inch  apart,  and  indicate  pounds 
pressure,  for  the  reason  that  the  descent 
of  one  inch  in  the  short  leg  causes  a  rise 
of  one  inch  in  the  long  leg,  making  a 
difference  in  the  level  of  the  mercury 
of  two  inches,  which  corresponds  to  one 
pound  pressure ;  that  is  to  say,  a  column 
of  mercury  two  inches  high,  and  having 
a  base  equal  in  area  to  one  square  inch, 
will  weigh  in  round  numbers  one  pound. 
In  making  a  gauge,  it  matters  not 
what  may  be  the  diameter  of  the  tube, 
but  whatever  it  may  be,  it  should  be  uniform  through- 
out, in"  order  that  the  indications  may  be  correct. 


T8  STEAM    AND    VACUUM    GAUGES. 

The  stick  that  is  put  in  the  long  leg,  when  there  is 
no  steam  on,  has  one  end  resting  on  the  mercury,  while 
the  other  stands  at  0.  This  stick  should  be  made  of 
some  very  light  wood — soft  white  pine  answers  the 
purpose  very  well,  with  the  lower  end  a  little  the 
largest,  in  order  to  have  a  good  bearing  on  the  mer- 
cury. 

To  convert  this  gauge  into  a  vacuum  gauge,  it 
would  be  necessary  only  to  connect  the  long  leg  to 
the  condenser,  and  attach  a  scale  to  the  short  leg  with 
the  lowest  number  commencing  at  the  top. 


CHAPTER  IV. 

CASUALTIES,    ETC. 

How  to  act  if  the  Eccentric  be  broken  in  an  irreparable 

manner. 

IF  there  be  two  paddle  engines  connected  at  an  angle 
of  90°,  connect  the  starting  bar  of  the  deranged  engine, 
by  means  of  a  line  and  guide  pulleys,  to  the  cross-tail, 
air-pump  beam,  air-pump  cross-head,  or  other  part 
having  motion  coincident  with  the  piston  of  the  other 
engine,  to  give  the  bar  motion  in  one  direction,  and 
attach  a  heavy  weight  to  it,  with  a  line  running  over 
a  pulley,  to  give  it  motion  in  the  opposite  direction. 

If  there  be  but  one  engine,  connect  by  similar 
means,  to  the  connecting  rod  of  the  deranged  engine, 
which  will  give  the  proper  motion. 

How  to  act  lulien  a  Steamer  springs   alealc  and  com- 
mences to  fill  rapidly. 

Put  on  immediately  all  bilge  injections  and  bilge 
pumps,  and  shut  off  all  other  injections.  If  they  do 
not  keep  the  water  down,  break  the  joints  on  the  bot- 
tom, or  side  injections,  and  allow  them  to  draw  water 
from  the  bilge,  taking  care  to  station  a  man  at  each 
one  to  prevent  any  thing  from  passing  in  that  would 
choke  the  valves. 

Vessels  are  sometimes  saved  from  foundering  by 
6 


80  CASUALTIES,    ETC. 

covering  the  leak  with  a  sail-cloth  passed  over  the 
bows  and  under  the  bottom. 

If  the  leak  be  a  large  one,  such  as  one  occasioned 
by  a  collision,  it  may  be  possible  to  force  a  mattress,  or 
something  of  that  nature,  into  it  from  the  outside. 

Hoio  to  proceed  when  all  the  feed  is  on  and  the  water 
does  not  rise  in  tlie  boilers. 

It  sometimes  happens  that  when  all  the  feed  is  on, 
and  the  feed  pumps  are  apparently  performing  their 
usual  duty,  the  water  does  not  rise  in  the  boilers,  but 
either  retains  its  level  at  the  time  the  feed  was  put  on, 
or  gradually  falls.  In  this  event,  one  of  two  things 
must  be  manifest — either  that  the  water  does  not  enter 
the  boiler,  or  if  it  does  enter,  is  escaping  through  some 
other  orifice.  The  first  thing,  therefore,  to  do,  is  to 
examine  the  check  valve  to  see  if  it  is  in  operation. 
This  can  be  done  by  applying  the  ear  to  the  chamber, 
to  ascertain  if  the  valve  rises  and  falls,  at  each  stroke 
of  the  pump,  and  also  by  applying  the  hand  to  the 
pipe,  immediately  below  the  check  valve,  in  order  to 
ascertain  if  it  be  cool.  If  these  are  found  to  be  all 
right,  examine  the  blow-off  cocks,  and  all  other  water 
connections  with  the  boilers,  to  ascertain  if  they  be 
closed ;  some  of  which,  in  all  probability,  will  be  par- 
tially open,  but  if  they  should  all  be  found  closed,  the 
pump  must  be  pumping  air  into  the  boilers  instead  of 
water.  The  next  step  would  therefore  be,  to  examine 
the  pump  and  induction  pipe,  in  order  to  ascertain  and 
stop  the  air  leak. 

Upon  examining  the  check  valve,  should  it  not  be 
found  in  operation,  the  next  step  would  be  to  examine 
the  pump,  to  see  if  it  was  hot ;  also  relief  and  pump 


CASUALTIES,    ETC.  81 

valves,  to  see  if  they  were  gagged ;  and  lastly,  the 
eduction  pipe,  to  see  if  it  were  burst — either  of  which 
causes  would  prevent  the  pump  from  delivering  water. 
A  feed  pump  may  get  hot  from  four  causes : 

First.  There  may  be  so  small  a  quantity  of  injec- 
tion water  used  as  to  cause  it,  when  delivered  to  the 
hot  well,  to  be  of  sufficiently  high  temperature  to  heat 
the  pump. 

Second.  Friction,  occasioned  from  muddy  water,  or 
tight  packing. 

Tkird.  The  check  and  delivery  valves  may  be 
caught  up  or  very  leaky,  allowing  the  hot  water  from 
the  boiler  to  run  back  to  the  pump. 

Fourth.  External  application  of  heat,  the  pump 
being  situated  near  the  boiler  or  other  hot  body. 

A  feed-pump  cannot  deliver  water  when  hot,  for 
the  reason  that  the  vapor  constantly  generated  within 
it,  by  its  elasticity  prevents  the  induction  valve  from 
opening  and  admitting  water. 

Should  the  feed  pipe  burst,  it  can  be  repaired  tem- 
porarily by  wrapping  it  with  canvas  coated  with  white 
lead ;  this  being  secured  by  strong  twine  or  marline, 
wound  closely  around  the  pipe  the  full  length  of  the 
canvas. 

Should  the  pipe  be  split  open  for  a  considerable 
distance,  it  might  first  be  closed  with  wood  or  iron 
clamps,  as  came  most  convenient,  before  applying  the 
canvas  and  twine. 

Foaming. 

Foaming,  or  pruning,  as  it  is  sometimes  termed,  is 
violent  ebullition  or  agitation  of  the  water,  occasioned 
by  an  undue  relation  of  temperature  between  the 
steam  and  water.  Thus,  supposing  a  large  quantity 


8 "2  CASUALTIES,    ETC. 

of  steam  to  be  suddenly  taken  from  the  boiler,  the 
pressure  of  steam  is  immediately  reduced  below  what 
is  due  to  the  temperature  of  the  water,  and  the  result 
is  a  sudden  rising  up  of  the  water  from  all  parts  of  the 
boiler.  Foam  can,  therefore,  be  defined  to  be  a  mix- 
ture of  steam  and  water.  Boilers  are  known  to  be 
foaming  when  the  water  does  not  come  out  of  the 
gauge  cocks  solid,  or  when  there  is  a  considerable  agi- 
tation of  the  water  in  the  glass  gauges. 

To  suppress  foaming,  put  on  a  strong  feed  arid 
blow  off,  cut  off  shorter  or  partially  close  the  throttle. 
Oil  or  melted  tallow,  injected  into  the  boilers  through 
the  feed  pumps,  will  also  prevent  foaming,  but  these 
are  somewhat  expensive  expedients. 

Boilers  constructed  with  insufficient  steam  room, 
are  most  likely  to  foam,  because  at  each  stroke  of  the 
piston  a  large  proportion  of  the  steam  is  taken  from 
the  boiler,  and  the  pressure  therefore  becomes  mate- 
rially reduced.  Boilers  also  constructed  in  such  a 
manner  as  to  prevent  the  easy  escape  of  steam  from 
the  surfaces  on  which  it  is  generated,  are  likely  to 
foam.  Thus,  supposing  there  be  a  large  amount  of 
heating  surface  on  the  crowns  and  other  parts  towards 
the  bottom  of  the  boiler,,  and  that  the  steam  generated 
on  these  surfaces  in  consequence  of  coming  in  contact 
with  the  flues,  tubes,  braces,  &c.,  can  find  but  a  com- 
paratively small  exit  to  the  surface  of  the  water, 
the  result  will  be,  that  where  it  does  escape,  it  will 
force  a  large  body  of  water  up,  mixing  it  with  the 
steam. 

To  cany  too  much  water  in  boilers  will  cause  them 
to  foam  by  reducing  the  steam  room.  Running  from 
salt  to  fresh  water,  or  vice  versa,  will  also  cause  foam- 
ing; in  the  former  case,  because  fresh  water  boils  at  a 


CASUALTIES,    ETC.  83 

lower  temperature,  but  a  satisfactory  explanation  of 
the  latter  case  appears  to  be  difficult  to  arrive  at.  The 
boilers  of  sea  steamers,  when  running  in  muddy  rivers, 
usually  foam  considerably. 

It  sometimes  occurs,  while  the  boilers  are  foaming 
badly,  that  the  engines  have  to  be  stopped  in  order  to 
take  soundings,  or  from  other  causes.  Now,  the  first 
thing  after  stopping  the  engines,  in  any  case,  is  always 
to  try  the  water;  for  it  will  mostly  always  be  found 
to  be  lower  when  the  engines  are  standing  still  than 
when  under  way,  but  when  the  boilers  are  foaming,  it 
is  of  the  highest  importance  to  try  immediately  the 
height  of  the  water,  for  as  the  foaming  ceases  after  the 
engines  are  stopped,  it  may  happen  that  the  water  has 
fallen  entirely  out  of  the  gauges  and  left  the  flues,  in 
which  event,  if  the  engines  were  going  to  be  started 
again  in  three  or  four  minutes,  the  better  plan  would 
be  to  open  the  safety  valve  to  keep  the  water  foaming, 
so  as  to  keep  the  flues  covered,  and  when  the  engines 
are  started  again  to  put  all  the  feed  on.  But  if  the 
engines  were  going  to  stand  still  for  a  considerable 
time,  blow  off  a  portion  of  the  steam,  if  it  be  too  high, 
dampen  the  fires  a  little,  and  put  on  the  auxiliary  feed. 

The  Condenser  heats. 

When  engines  are  standing  still,  it  sometimes 
occurs  that  the  condenser  gets  so  hot,  that  when  it 
becomes  necessary  to  start  again,  the  pressure  has  be- 
come so  great  in  it,  that  the  injection  water  will  not 
enter.  Leaky  steam  and  exhaust  valves  will  alone 
cause  this,  but  in  no  case  should  it  ever  be  allowed  to 
occur.  When  an  engine  begins  to  get  hot,  the  crack- 
ing noise  in  the  condenser,  and  about  the  foot  valves, 


84  CASUALTIES,    ETC. 

will  always  indicate  what  is  going  on,  time  enough  to 
check  it,  which  can  be  done  by  giving  a  little  injec- 
tion, and  causing  the  engines  to  make  two  or  three 
revolutions  back  and  forth.  If,  however,  the  engine 
should  become  too  hot  to  take  the  injection  water,  the 
only  plan  will  be  to  blow  through,  or  pump  water  into 
the  condenser  if  there  be  such  an  arrangement,  or  to 
cool  the  condenser  by  external  application  of  cold 
water. 

If  when  under  way  it  is  indicated  by  the  gauge 
that  the  engine  is  gradually  losing  its  vacuum,  apply 
the  hand  to  the  condenser,  in  order  to  ascertain  if  it 
be  getting  hot,  and  if  such  be  found  to  be  the  case 
give  a  little  more  injection ;  but  if  that  does  not  help 
the  cause,  give  more  still.  If  the  vacuum  continues  to 
grow  less,  the  probability  is  that  the  injection  pipe 
has  become  choked ;  in  which  event  shut  off  that  in- 
jection and  put  on  another.  Should  both  the  bottom 
and  side  become  choked,  inject  from  the  bilge.  Should 
the  bilge  injection  also  be  out  of  order,  the  engine  will 
have  to  be  stopped,  and  the  snifting  valve  secured 
down  (if  there  be  one)  while  the  injections  are  blown 
through  to  clear  them.  Sea  weed,  and  things  of  that 
nature,  sometimes  get  over  the  strainers  of  injection 
pipes,  preventing  the  entrance  of  water. 

Most  if  not  all  marine  engines  of  modern  construc- 
tion are  fitted  with  a  thermometer  to  the  hot  well,  to 
ascertain  the  temperature  of  the  water,  which  is  usually 
carried  from  100°  to  115°  Fahr.  This  instrument  is 
very  important,  in  order  to  maintain  an  even  temper- 
ature (the  sense  of  touch  of  the  engineer's  hand  not 
being  delicate  enough  for  that  purpose),  for  it  may 
often  occur  that  there  may  start  small  leaks  about  the 
condenser  and  exhaust  pipe  joints,  which  would  cause 


CASUALTIES,    ETC.  85 

a  decrease  in  the  vacuum,  and,  as  without  the  ther- 
mometer, the  first  impulse  would  be  to  give  more 
injection,  with  it  we  would  turn  our  attention  to  find- 
ing and  stopping  the  leak.  This  can  be  done  by  hold- 
ing a  lighted  candle  around  the  joints,  and  wherever 
there  is  a  leak  the  flame  will  be  drawn  in.  To  stop  it, 
mix  a  little  putty,  of  white  arid  red  lead,  and  apply  it 
to  the  crevice ;  the  presence  of  the  atmosphere  will 
force  it  in. 

Getting  under  way. 

When  lying  in  port,  where  the  steam  will  not  be 
required  for  at  least  four  or  five  days,  it  is  proper  that 
the  water  should  be  blown  or  pumped  out  of  the 
boilers,  and  a  portion  of  the  man  and  hand-hole  plates 
removed,  to  allow  a  circulation  of  air.  When,  there- 
fore, the  order  is  given  to  get  up  steam,  the  first  thing 
is  to  see  that  all  these  plates  are  put  on,  and  the  joints 
properly  made,  and  this  duty  should  receive  the  direct 
superintendence  of  the  engineer  having  charge  of  the 
same ;  for  should  any  one  of  them  leak  badly  after  the 
steam  is  raised,  the  departure  of  the  ship  might  be  de- 
layed some  hours  in  consequence.  After  this  duty  has 
been  properly  attended  to,  open  the  blow-off  cocks  and 
run  the  water  up  in  the  boilers  to  the  proper  level,  or, 
if  the  boilers  are  so  situated  that  the  water  will  not  run 
up  high  enough,  finish  the  supply  with  the  hand 
pumps,  wood  the  furnaces  while  the  water  is  entering 
the  boiler,  and  when  the  proper  height  of  water  is 
attained  start  the  fires.  If  it  be  important  to  raise 
steam  quickly,  start  the  fires  as  soon  as  water  is  dis- 
covered in  the  gauges,  continuing  the  supply  while  the 
fires  are  burning.  As  a  small  quantity  of  finely  split 
wood,  with  a  little  shavings  or  oily  waste  placed  in 


86  CASUALTIES,    ETC. 

the  mouth  of  the  furnaces,  is  all  that  is  necessary  to 
start  the  fires,  the  back  part  of  the  furnaces,  particu- 
larly in  boilers  with  inferior  draft,  should  be  covered 
with  a  layer  of  coal  to  keep  out  the  cold  air. 

In  raising  steam  it  has  been  the  custom  to  recom- 
mend that  the  valves  of  the  engine  be  blocked  open, 
so  as  to  allow  the  heated  air  from  the  boilers  to  pass 
in  and  warm  up  the  engine  before  steam  begins  to  be 
generated ;  but  as  in  many  cases  this  is  attended  with 
considerable  trouble,  and  as  the  advantages  to  be  de- 
rived from  it  are  very  small,  it  hardly  appears  to  the 
author's  mind  to  "  pay."  The  safety  or  vacuum  valve 
should,  however,  be  kept  open  until  steam  begins  to 
form,  in  order  to  let  the  heated  air  escape.  The  strain 
upon  boilers  being  from  the  inside,  they  are  con- 
structed and  braced  with  the  special  view  of  with- 
standing this  strain,  many  of  the  braces  being  entirely 
useless  in  sustaining  a  pressure  from  without ;  marine 
boilers  are  therefore  fitted  with  a  small  valve  opening 
inwards,  and  weighted  so  as  to  open  and  admit  air 
whenever  the  pressure  from  within  falls  to  about  five 
pounds  per  square  inch  below  the  atmosphere.  These 
valves  are  called  differently  by  different  parties,  as 
follows :  vacuum  valve,  air  valve,  reverse  valve,  &c. 

After  steam  has  been  raised  to  3  or  4  Ibs.,  the 
engine  should  then  be  blown  through  and  warmed  up, 
and  after  sufficient  steam  is  raised  to  move  the  piston, 
the  engine  should  be  turned  over  two  or  three  times, 
to  see  that  every  thing  is  right,  before  reporting 
ready. 

On  Coming  into  Port. 

After  the  engines  are  no  longer  needed,  before 
hauling  the  fires,  after  a  long  run,  it  would  be  well 


CASUALTIES,    ETC.  87 

to  try  the  pistons  and  valves,  in  order  to  ascertain 
if  they  be  leaky.  To  try  the  piston,  open  the  water 
valve  on  one  end  of  the  cylinder  and  the  steam 
valve  on  the  opposite  end ;  if  the  piston  leaks,  the 
steam  will  escape  through  the  water  valve.  To  ascer- 
tain if  the  steam  valves  leak,  open  the  water  valves  on 
both  ends  of  the  cylinder.  To  ascertain  if  the  exhaust 
valves  leak,  open  the  steam  valves  and  any  cock 
in  the  exhaust  side  of  the  steam  chest  or  exhaust 
pipes. 

While  under  way  it  may  be  discovered  that  there 
is  a  slight  thump  in  the  engine  when  passing  one  or 
or  the  other  or  both  centres,  and  the  indicator  hav- 
ing been  applied  shows  the  usual  lead,  the  inference 
is  that  some  part  of  the  working  engine  is  loose ;  it  is 
important,  therefore,  to  find  out  what  it  is  on  coming 
into  port.  To  do  this  place  the  engine  on  the  centre, 
and  give  the  piston  steam  suddenly  by  raising  and 
lowering  the  starting  bar ;  observe  closely  the  cross- 
head,  crank-pin,  main-shaft,  and  other  main  connections, 
to  see  where  the  jar  is.  Should  it  not  be  discovered 
after  this,  jam  the  cross-head  fast,  so  as  to  prevent  the 
slightest  motion,  and  then  give  steam  as  before,  in 
which  event,  if  the  thump  be  still  felt,  the  piston  will 
doubtless  be  found  to  have  worked  a  little  loose. 

If  it  be  the  intention  to  remain  in  port  several 
days,  before  hauling  the  fires,  sufficient  steam  should 
be  raised,  if  the  boilers  be  capable  of  bearing  the 
pressure,  to  blow  all  the  water  out  of  the  boilers.  After 
the  boilers  become  cool,  the  hand-hole  plates,  over  the 
furnaces  particularly,  should  be  taken  off,  to  examine 
the  crowns,  where  the  greater  amount  of  scale  will  be 
found  deposited,  and  from  which  we  can  judge  if  the 
boilers  require  scaling.  Mere  dampness  in  boilers  is 


88  CASUALTIES,    ETC. 

found  to  be  injurious,  by  occasioning  a  rapid  oxida- 
tion, and  in  order  to  prevent  this,  one  or  two  hand- 
hold plates  should  be  taken  off  the  bottom  of  the 
boilers,  in  order  to  let  the  water  drain  out  dry.  It 
would  be  well  also  to  remove  a  man-hole  plate  from 
the  top  of  the  boilers  to  allow  a  circulation  of  air. 
If  these  things  cannot  be  done  it  will  be  better  to  keep 
the  boilers  filled  with  water,  rather  than  a  small 
quantity  in  the  bottoms.  In  damp  climates,  such  as 
the  Isthmus  of  Panama,  light  fires  should  be  made  in 
the  ash-pits  occasionally. 

Scaling  Boilers. 

Notwithstanding  the  water  in  the  boilers  is  not 
allowed  to  exceed  in  density  If  to  2  per  saline  hydrom- 
eter, it  will  be  found  after  a  time  that  a  quantity  of 
scale,  composed  principally  of  lime,  has  accumulated 
on  the  crown  sheets,  tubes  or  flues,  and  other  parts  of 
the  boiler.  If  this  be  allowed  to  remain  the  metal 
will  become  overheated  and  burned ;  it  becomes  ne- 
cessary, therefore,  to  remove  it,  which  can  be  alone 
done  by  mechanical  means.  Sharp-faced  "  scaling 
hammers"  can  be  used  to  knock  the  scale  off  those 
places  that  are  within  the  arm's  reach,  and  long  bars 
flattened  at  both  ends,  and  sharpened,  called  "  scaling 
bars,"  will  knock  it  off  the  more  remote  parts.  In  the 
Martin  tubular  boiler,  which  is  accessible  in  every 
part,  it  is  only  necessary  to  condense  the  steam  in  the 
boilers  for  a  day  or  so  after  the  ship  comes  to  anchor ; 
this  will  soften  the  scale  so  that  a  gang  of  men  may  be 
put  into  them  as  soon  as  the  man-hole  plates  are  re- 
moved, and  scrape  off  all  of  it  in  a  few  hours.  The 
scale,  however,  must  never  be  allowed  to  exceed  the 
thickness  of  writing  paper. 


COMING   TO    ANCHOE.  89 

It  has  been  proposed  in  some  quarters  to  heat  the 
tubes  or  flues  by  burning  shavings,  or  some  other  such 
substance  in  them,  and  then  to  cool  them  off  suddenly 
by  pumping  cold  water  upon  them,  the  sudden  con- 
traction causing  the  scale  to  crack  off.  This  plan,  how- 
ever, to  our  mind,  does  not  deserve  much  favor,  and 
never  should  be  resorted  to,  if  the  scale  can  be  reached 
in  any  other  manner,  for  the  production  of  leaks  will 
mostly  always  be  the  result. 

It  is,  however,  hoped  that  engineers  will 'soon  be 
relieved  from  this  duty,  and  steamer  owners  benefited 
by  the  introduction  of  fresh  water  condensers  into  all 
sea  steamers. 

Preparatory  to  coming  to  Anchor,  or  securing  to  the 

Wharf. 

Fifteen  or  twenty  minutes  before  coming  to  anchor, 
or  making  fast  to  the  wharf,  the  chief  engineer  should 
be  informed  of  the  fact  by  the  officer  of  the  deck,  or 
some  other  person  informed  on  the  matter,  so  that  the 
fires  can  be  allowed  to  burn  down,  and  the  pressure  of 
steam,  permitted  to  fall  to  such  an  extent  that  the 
necessity  for  blowing  off  is  avoided.  By  this  means 
the  great  nuisance  of  blowing  off  steam  is  not  only 
obviated,  but  there  is  a  considerable  saving  in  fuel, 
the  fires  being  permitted  to  burn  down  sufficiently  low 
to  supply  only  the  amount  of  steam  required  while 
working  the  engines  by  hand,  rendering  it  much  easier 
also  on  the  firemen  (whose  duties  on  any  occasion  are 
arduous  enough)  by  having  a  very  light,  instead  of  a 
very  heavy  fire  to  haul. 

In  coming  to  anchor  it  is  usually  well  to  pump  a 
little  extra  water  into  the  boiler,  so  as  to  insure  a 
proper  supply  while  operating  the  engines  by  hand. 


90  THE   FIRES    WHILE   UNDER    WAY. 

When  it  is  desired  to  raise  steam,  the  order  from 
the  captain  should  always  be  what  time  it  is  intended 
to  get  underway,  leaving  to  the  discretion  of  the  chief 
engineer  to  start  the  fires  at  such  time  as  he  may  con- 
sider proper,  in  order  to  secure  steam  and  every  thing 
ready  at  the  proper  time. 

Regarding  the  Fires  while  under  Way. 

Small  as  this  may  appear  in  the  eyes  of  one  not 
practically  conversant  with  the  management  of  the 
steam  engine,  it  is  one  of  the  most  important  things 
that  the  engineer  is  called  upon  to  regulate :  on  the 
one  hand,  that  a  proper  and  uniform  supply  of  steam 
is  maintained,  and  on  the  other,  that  more  fuel  is  not 
consumed  than  is  actually  necessary  to  produce  the  re- 
sult. Different  fuels  and  differently  constructed  boil- 
ers require  the  fires  to  be  regulated  in  a  different 
manner,  and  notwithstanding  the  repeated  efforts,  the 
adoption  of  specific  rules,  which  shall  apply  alike  to 
all,  is  positively  absurd.  A  few  general  hints,  how- 
ever, touching  the  leading  features,  may  be  useful  to 
those  who  have  not  had  much  experience  in  this  mat- 
ter, but  they  must  bear  in  mind,  nevertheless,  that 
actual  service  and  observation  for  themselves,  will 
alone  make  them  proficient,  no  matter  how  well  they 
may  understand  the  chemistry  of  coal,  or  the  natural 
laws  governing  the  combustion  of  matter. 

The  proper  supply  of  atmospheric  air,  and  the 
proper  time  for  the  combustion,  are  the  important  ele- 
ments in  the  consumption  of  coal.  A  slow  rate  of 
combustion,  and  a  moderate  draft,  always  producing  a 
better  evaporative  result,  than  when  the  fires  are  urged, 
occasioning  them  to  be  more  rapid;  and  hence,  on 


THE    FIEES    WHILE   TJKDEK   WAY.  91 

no  occasion,  should  "  blowers  "  be  resorted  to,  if  the 
proper  supply  of  steam  can  be  maintained  without 
them. 

The  fire  should  be  spread  uniformly  all  over  the 
grate  bars,  and  in  the  use  of  bituminous  coal,  should 
be  from  6  to  8  inches  in  thickness,  but  with  anthracite 
coal,  4  or  5  inches  will  be  thick  enough.  So  long  as 
the  ash  pit  remains  bright,  there  is  no  necessity  for 
slicing  or  stirring  up  the  fire,  but  whenever  the  spaces 
between  the  bars  become  choked  with  clinker,  or 
ashes,  it  will  be  indicated  by  the  darkness  in  the  ash 
pit,  and,  if  burning  bituminous  coal,  a  slice  bar  should 
be  run  in  through  the  stoke  holes  or  furnace  doors  to 
break  up  the  fire  and  clear  out  the  air  spaces.  A  pick 
applied  from  below  is  also  very  useful  in  this  respect. 
In  the  use  of  anthracite  coal  the  pick  alone  should  be 
used ;  the  breaking  up  of  the  surface  of  such  fire, — as 
it  does  not  amalgamate  or  run  together,  forming  a 
crust  like  the  bituminous, — prevents  the  regular  uni- 
form combustion  by  allowing  too  much  air  to  enter 
among  the  disturbed  parts  of  the  coal,  it  requiring 
considerable  time  for  them  again  to  unite  in  regular 
ignition  after  being  once  disturbed.  It  is  very  impor- 
tant that  no  part  of  the  grate  bars  be  left  bare,  as  the 
admission  of  cold  air,  through  such  space,  deadens  the 
fire,  and  cools  the  flues.  It  has  been  ascertained  of 
late,  that  better  results  are  obtained  by  admitting  air 
through  a  number  of  small  holes  in  the  furnace  doors, 
on  the  plan  of  W.  Wye  Williams,  Esq.,  of  England. 

No  two  furnaces  should  be  fired  at  the  same  time ; 
the  fresh  coal  of  the  one  should  be  fairly  ignited  before 
a  new  supply  is  added  to  another,  in  order  to  keep  a 
regular  supply  of  steam.  Anthracite  coal  requires  less 
frequent  firing  than  bituminous,  but  with  either,  the 


92  THE   FIRES    WHILE    UNDER    WAY. 

coal  should  not  be  thrown  upon  any  particular  part 
of  the  furnace,  but  uniformly  all  over  it.  Before 
firing  with  bituminous  coal,  it  is  well  to  break  up  the 
upper  crust  of  the  fire,  which  sometimes  amalgamates 
so  closely  as  to  exclude  the  proper  supply  of  air.  The 
trouble  with  most  firemen  is,  that  they  are  disposed  to 
heap  their  fires  too  much,  particularly  in  front,  some- 
times half  way  to  the  crowns  ;  this  they  do  for  three 
reasons :  first,  because  they  suppose  the  larger  the  fire 
the  greater  the  supply  of  steam ;  second,  the  more 
coal  there  is  piled  in  at  one  time,  the  less  frequent 
they  will  have  to  fire  ;  and  third,  it  requires  much  less 
labor  to  shovel  the  coal  into  the  mouth  of  the  furnace, 
than  to  supply  it  uniformly,  all  over  the  grates.  No 
coal  larger  than  one's  fist  should  be  allowed  to  enter 
the  furnace,  nor  in  cleaning  the  fires,  should  more  than 
one  be  cleaned  at  the  same  time,  which  should  be  done 
at  stated  intervals,  unless  it  so  happens,  that  they  all 
or  many  of  them,  have  got  so  dirty  that  a  further  sup- 
ply of  coal  is  useless,  when  the  engine  can  be  throttled 
off  a  little  while  the  cleaning  is  going  on.  In  cleaning 
anthracite  fires,  care  should  be  taken  not  to  reduce 
them  too  low,  otherwise  they  will  take  a  long  time  to 
recover. 

In  cleaning  fires,  as  well  as  when  supplying  them, 
the  furnace  doors  should  not  be  kept  open  longer  than 
necessary,  admitting  an  undue  supply  of  cold  air ;  and 
the  party,  therefore,  who,  performing  his  'duty  as  well, 
does  it  the  quickest,  is  the  best  fireman. 

The  slower  a  steamer  runs  the  greater  distance  she 
will  perform  with  the  same  amount  of  fuel,  provided 
she  has  not  an  adverse  tide  or  head  winds  to  contend 
with ;  with  men-of-war,  therefore,  it  often  occurs  that 
the  saving  of  fuel  is  a  more  important  consideration 


PATCHING    BOILERS.  93 

than  high  speed,  and  for  this  reason  the  consumption 
of  coal  is  reduced  far  below  what  would  be  required 
to  keep  the  vessel  up  to  her  maximum  speed.  This 
can  be  done  in  two  ways :  either  by  shutting  off  a  por- 
tion of  the  furnaces  entirely,  by  shutting  the  ash  pit 
doors  and  closing  up  the  cracks  around  them  with  wet 
ashes,  or  else  reducing  the  quantity  of  coal  consumed 
in  each,  by  covering  the  back  part  of  the  grates  with 
a  thick  layer  of  ashes.  When  the  diminution  in  the 
quantity  of  coal  is  not  very  large,  this  latter  plan  is 
the  better,  by  retaining  the  original  heating  surface  at 
the  same  time  that  the  combustion  of  coal  is  allowed 
to  go  on  very  slowly,  an  end  very  desirable  to  secure. 
When,  however,  the  reduction  in  coal  is  very  consid- 
erable, some  of  the  furnaces  can  be  shut  off,  while  the 
back  ends  of  the  grates  of  the  remainder  can  be  kept 
covered  with  ashes.  Men-of-war  sometimes  proceed  at 
half  or  less  speed,  and  as  a  large  extent  of  boiler  sur- 
face occasions  considerable  loss  from  radiation,  in  such 
cases  it  will  be  more  economical  to  shut  off  some  of  the 
boilers  and  continue  with  a  moderate  supply  of  fuel  in 
the  remainder.  The  furnaces  and  ash  pits  of  the  boil- 
ers shut  off  should  be  closed  tightly,  to  prevent  cold 
air  from  passing  in  to  cool  the  surfaces  of  the  other 
boilers,  or  to  injure  the  draft. 

After  a  boiler  is  shut  off,  the  steam  should  not  be 
allowed  to  escape,  but  to  remain  in  it  and  condense,  to 

freshen  the  water. 

• 

% 

Patching  Boilers. 

Inasmuch  as  all  things  constructed  by  human  hands 
are  liable  to  decay,  steam  boilers  are  not  exempt  from 
this  infallible  law ;  they  therefore  frequently  require 
to  be  patched,  new  stay  bolts  and  braces  to  be  put  in, 


94  PATCHING    BOILERS. 

old  rivets  cut  out  and  replaced  with  new  ones,  <fec.  In 
patching  boilers,  wherever  the  defective  part  can  be 
reached  so  as  to  work  at  it  well,  it  is  best  to  cut  it  out 
and  rivet  a  patch  on,  calking  the  seams ;  but  as  this 
cannot  always  be  done,  the  most  common  practice  is 
to  put  a  patch  over  the  defective  part,  securing  it  with 
bolts  and  nuts,  or  tap  bolts,  and  making  the  joint  with 
stiff  putty,  composed  of  white  and  red  lead,  and  a 
small  quantity  of  fine  iron  borings.  A  piece  of  sheet 
lead  fitted  over  the  place  to  be  patched,  will  answer 
for  the  pattern  to  make  the  patch  by,  which,  however, 
before  the  joint  is  made,  should  be  fitted  snugly  to  its 
place  while  hot. 

Owing  to  imperfection  in  the  iron,  small  cracks  are 
sometimes  discovered  in  the  flues  or  other  parts  of  the 
boiler,  subject  to  a  high  temperature.  Should  these 
not  be  more  than  two  or  three  inches  in  length,  they 
can  be  stopped  by  drilling  holes  and  putting  in  three 
or  four  small  rivets,  hammering  the  heads  well  down 
so  as  to  cover  the  crack. 

A  leaky  stay-bolt,  or  rivet,  has,  like  the  toothache, 
but  one  sure  remedy,  and  that  one  is  to  cut  it  out  and 
put  in  a  new  one. 

In  cutting  out  a  stay-bolt  fitted  with  a  socket,  the 
latter  can  usually  be  saved  and  retained  in  its  place, 
ready  to  receive  another  bolt ;  but  sometimes  a  screw 
bolt  is  cut  out  which  has  to  be  replaced  with  a  socket 
bolt,  and  as  this  may  be  in  such  part  of  the  boiler 
which  cannot  be  reached  by  the  arm,  or  tongs,  a  very 
good  plan  to  get  the  socket  in  its  place,  is  to  pass  a 
string  through  both  holes  and  secure  the  ends,  drop- 
ping the  centre  down  and  hauling  it  out  through  a 
hand  hole ;  cut  the  string  in  two,  pass  the  ends  through 
the  socket,  join  them  together  again,  and  haul  the 


FLUES   AND    ASH    PITS.  95 

socket  to  its  place.  In  the  fitting  of  sockets,  it  is  very 
important  that  they  should  be  the  exact  distance  be- 
tween the  sheets,  with  the  ends  filed  square,  otherwise 
the  sheets  will  be  drawn  out  of  shape. 

Sweeping  Flues. 

One  of  the  most  disagreeable  parts  of  the  duties  is 
that  of  cleaning  flues,  from  the  fact  of  its  dirtying 
every  thing  round  about  or  in  the  vicinity  of  the  boil- 
ers, the  slightest  draft  being  sufficient  to  waft  the  light 
dry  ashes  in  every  direction.  A  little  water  sprinkled 
on  them  before  they  are  hauled  out  of  the  connections 
or  smoke-boxes  will  prevent  this  in  a  measure,  the 
damper  and  ash-pit  and  furnace  doors  being  closed,  to 
prevent  the  men  from  being  suffocated  who  go  inside. 
The  lower  flues,  particularly,  are  apt  to  leak  a  little, 
and  the  salt  water,  mixing  with  the  ashes,  forms  a  solid 
mass,  which  can  only  be  removed  by  being  cut  out, 
the  flue  brush  being  of  no  avail.  The  hammer  and 
chisel,  and  long,  sharp-pointed  bars,  and  sledge,  are 
best  adapted  to  the  purpose.  In  the  use  of  these 
instruments,  care  should  be  taken  that  they  be  not 
driven  through  the  metal  or  under  the  seams. 

Ash  Pits. 

The  ash  pits  should  be  cleaned  out  every  watch, 
and  the  ashes  thrown  overboard,  picking  out  first  any 
lumps  of  coal  that  may  have  fallen  among  the  ashes. 
When  not  running  at  full  speed,  a  portion  of  the  cin- 
ders may  be  thrown  upon  the  fires  again,  after  damp- 
ing them  with  a  little  water.  So  also  should  fine 
bituminous  coal  be  dampened  before  being  supplied 
to  the  furnaces,  the  arguments  to  the  contrary  not- 
7 


96  STAYS    AND    GEATE    BARS. 

withstanding ;  for  though  it  does  take  a  little  heat  from 
the  fire  to  evaporate  the  water  mixed  with  the  coal,  a 
saving  is  effected1,  by  preventing  the  coal  from  being 
drawn — particularly  in  boilers  with  strong  draft- 
through  the  flues  and  lodged  in  the  connections,  or  out 
of  the  smoke-pipe.  No  more  water,  however,  should 
be  put  on  the  coal  than  just  sufficient  to  dampen  it. 

Smok&pipe  Stays 

Require  to  be  looked  to  occasionally,  when  made 
of  rope,  as  they  grow  a  little  slack  from  time  to  time. 
These  should  always  be  adjusted  while  the  pipe  is  hot ; 
otherwise,  if  they  be  set  up  while  the  pipe  is  cool,  the 
expansion  after  it  becomes  heated  will,  in  all  proba- 
bility, "  carry  "  either  the  stays  themselves  away,  or 
the  band  securing  them  to  the  pipe.  In  a  gale  of 
wind,  when  the  ship  is  rolling  heavily,  these  stays 
should  be  looked  to,  in  order  to  tighten  any  of  them 
that  may  have  become  slack,  so  as  to  throw  the  strain 
alike  on  all.  Hemp  rope  is  a  very  inferior  article  for 
such  purpose  as  stays  for  smoke  pipes,  and  we  can  see 
no  good  reason,  unless  it  be  prejudice,  (which  is  always 
a  good  reason  to  those  under  such  influence,)  why  it 
has  been  so  long  retained.  Good  wire  rope  looks  bet- 
ter, is  cheaper,  and  will  last  a  great  deal  longer,  and 
requires  much  less  attention. 

Grate  Bars,  &c. 

When  fitted  new,  are  usually  allowed  plenty  of 
play,  both  fore  and  aft  and  sideways,  to  allow  for  ex- 
pansion after  they  become  heated.  The. spaces  at  the 
end  of  the  bars,  however,  become  choked  up  with 
ashes,  which  become,  by  and  by,  so  hard  as  to  form 


BROKEN   AIR-PUMP.  9*7 

almost  a  solid  mass,  defeating  the  objects  for  which 
they  were  left.  These  spaces,  therefore,  in  port, 
should  be  cleaned  out  occasionally. 

Ash  pits,  in  port,  should  also  be  well  cleaned  and 
painted,  to  prevent  oxidation.  At  sea,  no  water  should 
be  thrown  into  them  upon  the  ashes,  but  they  should 
be  kept  as  dry  as  possible.  With  these  precautions, 
they  will  last  as  long  as  other  parts  of  the  boiler. 
Boilers  unused  for  any  considerable  time  should  be 
kept  dry  of  water,  and  have  fires  made  occasionally  in 
the  ash  pits,  to  evaporate  all  interior  deposit  of  damp- 
ness— the  neglect  of  this  precaution  is  the  sole  cause 
of  the  oxidation  and  deterioration  of  all  boilers  when 
not  in  use. 

Broken  Air-Pump. 

Should  the  air-pump  become  broken  in  an  irrepar- 
able manner,  and  the  engine  be  a  single  one,  there  is 
but  one  thing  that  can  be  done,  and  that  is  to  work 
non-condensing.  If  there  be  two  engines,  we  have 
three  resorts:  to  work  the  broken  engine  non-conden- 
sing, to  disconnect  from  the  crank  pin  and  proceed 
with  one  engine,  or,  if  there  be  facilities  on  board,  to 
join  the  exhaust  of  both  engines  with  a  pipe,  and  use 
one  air-pump  and  one  condenser  for  both  engines. 
This  latter  plan  was  tried  very  successfully  for  a  short 
run  on  board  the  U.  S.  Steam  Frigate  "  Powhatan," 
on  the  China  station,  in  the  summer  of  1855.  Peculiar 
facilities  were,  however,  offered  in  this  case,  as  the  ex- 
haust side  pipe  of  each  engine  had  a  man-hole  in  it,  to 
which  the  connecting  pipe  was  joined. 

In  running  under  such  circumstances,  care  should 
be  taken  not  to  overload  the  air-pump. 


98  CYLINDER    HEAD    AND    SELECTION    OF    COAL. 

Broken  Cylinder  Head. 

Water  may  be  worked  over  into  the  cylinder  sud- 
denly, from  boilers  foaming  badly,  or  otherwise,  faster 
than  it  can  escape  through  the  water  valves,  and  being 
nearly  non-compressible,  something  must  give  way,  the 
cylinder  head,  or  bottom,  being  the  most  likely  thing  to 
go.  In  such  an  event,  if  there  be  a  spare  one  on  board, 
put  it  on ;  if  not,  while  the  old  one  is  being  repaired,  if  it 
be  reparable,  the  following  plan  can  be  resorted  to ; 
Disconnect  the  steam  and  exhaust  valves  from  the 
damaged  end  of  the  cylinder,  if  the  engine  be  fitted 
with  poppet  valves,  and  let  the  atmospheric  pressure 
force  the  piston  in  one  direction,  the  steam  being  used 
for  the  opposite  direction.  Should  the  engine  be  fitted 
with  a  slide  valve,  close  up  the  opening  into  the  dam- 
aged end  of  the  cylinder,  by  fitting  in,  steam-tight  and 
in  a  substantial  manner,  a  block  of  soft  wood.  This 
should  not,  however,  be  resorted  to,  except  in  cases  of 
great  emergency.  Cylinder  heads  should  have  man- 
hole plates  of  less  strength  than  the  heads  ;  this  would 
prevent  the  destruction  of  heads  in  all  cases. 

The  selection  of  Coal. 

The  kinds  and  qualities  of  coals  are  so  varied  that 
no  general  rules  can  be  given  for  their  selection,  but 
there  is  one  point,  however,  which  we  think  will  not 
be  disputed,  and  that  one  is,  whenever  there  is  a 
choice,  the  only  sure  plan  is  to  select  the  best ;  for, 
though  its  first  cost  may  be  a  little  more,  it  will  prove 
to  be  the  cheapest  in  the  end.  What  economy  is  there 
in  purchasing  one  coal  because  it  can  be  obtained  10 


SAFETY    VALVE.  99 

or  15  per  cent,  cheaper  than  another,  when  there  will 
be  burned,  to  produce  the  same  effect,  from  20  to  25 
per  cent,  more  than  would  be  burned  by  the  better 
kind  ?  Yet  this  is  a  thing  of  daily  occurrence.  But, 
regardless  of  the  money  view,  there  are  other  disad- 
vantages attending  the  use  of  the  inferior  coal.  From 
the  fact  of  there  being  more  burne"d,  the  firemen  have 
more  to  supply  to  the  furnaces,  and  it  requires,  on 
their  part,  greater  care  and  attention  to  keep  the  fires 
in  good  order  ;  thus  imposing  extra  duty  on  a  portion 
of  the  ship's  crew  whose  energies  are  usually  overtaxed. 
Besides,  to  convey  the  vessel  a  given  distance,  an  extra 
quantity  has  to  be  taken  on  board,  which,  in  the  case 
of  merchant  ships,  diminishes  their  freight  capacity,  or, 
in  war  ships,  lumbers  the  deck  with  a  useless  number 
of  bags. 

Some  boilers  are  best  adapted  to  bituminous  coals, 
others  to  anthracite,  and  the  one  or  the  other  of  these 
coals  which  should  be  selected,  depends  upon  the  cir- 
cumstances, therefore,  for  which  they  are  intended. 

In  the  selection  of  coals,  it  is  an  object  to  obtain 
those  free  as  possible  from  earthy  impurities.  Slate, 
and  such  like  matter,  is  to  be  avoided.  Sulphur  in 
bituminous  coals  makes  them  the  more  liable  to  spon- 
taneous combustion.  So  also  receiving  them  on  board 
wet  will  endanger  spontaneous  ignition.  Coals  which 
have  been  exposed  a  long  while  to  the  rays  of  the  sun, 
particularly  in  tropical  climates,  undergo  a  gradual 
decay,  reducing  their  evaporative  qualities. 

Safety   Valve. 

Steam,  when  once  commencing  to  blow  off,  will 
not  cease  when  the  pressure  has  fallen  to  the  pressure 


100  SAFETY    VALVE. 

due  to  that  for  which  the  safety  valve  is  loaded,  but 
will  continue  to  blow-off  until  the  pressure  has  fallen 
some  pounds  below  this.  This  is  owing  to  the  increased 
area  which  the  steam  has  to  act  upon  when  the  valve 
is  open  over  what  it  has  when  the  valve  is  closed,  oc- 
casioned by  the  bevel  of  the  valve  face.  In  a  heavy 
sea,  the  safety  valve  may  be  forced  open  for  a  short 
time,  even  when  the  pressure  is  below  that  for  which 
the  valve  is  loaded,  by  the  oscillation  of  the  ship. 


CHAPTER  V. 

MISCELLANEOUS. 

TJie  Theory  of  the  Paddle  Wheel  /  the  Radial  compared 
with  the  Feathering  Wheel. 

To  all  those  whose  minds  have  a  tendency  to  probe 
beyond  the  superficial  crust  of  any  thing  that  may  be 
presented  to  their  consideration,  the  theory  of  the  ac- 
tion of  the  paddle  wheel  on  the  water  must  be  one  of 
interest,  and  any  thing,  therefore,  tending  to  make 
this  subject  the  more  clear,  cannot  fail  to  receive  the 
proper  attention  and  a  careful  perusal. 

In  regard  to  the  paddle  wheel,  many  theories  have 
been  advanced,  some  of  them  so  positively  absurd  that 
it  is  difficult  for  us  to  conceive  how  they  ever  found 
their  way  into  print.  Even  in  reference  to  the  subject 
of  centre  of  pressure  of  the  paddles,  such  rules  as  the 
following  have  been  put  forth  from  quarters  to  which 
we  should  have  looked  for  more  correct  information : 

"  The  circle  described  by  the  point  whose  velocity 
equals  the  velocity  of  the  ship,  is  called  the  rolling  cir- 
cle, and  the  resistance  due  to  the  difference  of  velocity 
of  the  rolling  circle  and  the  centre  of  pressure  is  that 
which  operates  in  the  propulsion  of  the  vessel."  *  *  * 

Rule :  "  From  the  radius  of  the  wheel  subtract  the 
radius  of  the  rolling  circle,  to  the  remainder  add  the 
depth  of  the  paddle  board,  and  divide  the  fourth 


102  THEORY    OF   THE   PADDLE    WHEEL. 

power  of  the  sum  by  four  times  the  depth ;  from  the 
cube  root  of  the  quotient  subtract  the  difference  be- 
tween the  radii  of  the  wheel  and  the  rolling  circle,  and 
the  remainder  will  be  the  distance  of  the  centre  of 
pressure  from  the  upper  edge  of  the  paddle.  The 
diameter  of  the  rolling  circle  is  very  easily  found,  for 
we  have  only  to  divide  5280  times  the  number  of  miles 
per  hour  by  60  times  the  number  of  strokes  per  min- 
ute, to  get  an  expression  for  the  circumference  of  the 
rolling  circle,  or  the  following  rule  may  be  adopted : 
Divide  88  times  the  speed  of  the  vessel  in  statute  miles 
per  hour,  by  3.1416  times  the  number  of  strokes  per 
minute ;  the  quotient  will  be  the  diameter  in  feet  of 
the  rolling  circle." 

Now,  then,  I  suppose  no  one  who  has  given  the  sub- 
ject the  slightest  attention  would  imagine,  for  one  mo- 
ment, that  so  long  as  the  immersion  remained  constant,  a 
difference  in  the  slip  of  a  common  radial  wheel  would 
make  a  difference  in  the  centre  of  pressure  of  the  pad- 
dles ;  yet  if  any  one  will  take  the  trouble  to  work  out 
the  centre  of  pressure  of  any  wheel  by  the  above  rule 
with  different  slips,  he  will  find  the  centre  of  pressure 
continually  changing.  To  suppose  such  a  thing  to  be  true 
would  be  as  absurd  as  to  suppose  the  centre  of  pres- 
sure of  a  plank  immersed  vertically  in  a  stream  moving 
at  the  rate  of  10  miles  per  hour,  to  be  in  a  different 
place  from  what  it  would  be  should  the  stream  move 
at  the  rate  of  5  miles  per  hour. 

We  have  thought  it  advisable,  therefore,  to  go  into 
this  subject  the  more  fully,  and  give  the  following  as 
an  illustration  of  our  views : 

It  is  generally  admitted  that  the  total  loss  of  effect, 
or  power,  in  the  common  radial  wheel,  is  the  sum  of 
the  losses  of  the  oblique  action  on  the  water  and  the 


THEORY  OF  THE  PADDLE  WHEEL. 


103 


slip.  The  former  is  calculated  by  taking  the  mean  of 
the  squares  of  the  sines  of  the  angle  of  incidence  at 
which  the  paddles  strike  the  water,  or  which  is  the 
same  thing,  the  means  of  the  squares  of  the  cosines  of 
the  angles  of  the  arm  and  water ;  for  one  angle  is  the 
complement  of  the  other.  This  will  appear  plain  from 
an  inspection  of  figure  1.  A  C  is  the  arm,  making 

FIG,  I 


an  angle  at  C,  with  the  vertical  line  C  A' ;  A  B,  the 
breadth  of  the  paddles,  and  E  F,  the  surface  of  the 
water.  Now,  it  is  manifest,  that,  inasmuch  as  the  ves- 
sel is  moving  in  a  horizontal  direction,  the  line  B  D  at 
right  angles  to  that  direction,  represents  the  only  por- 
tion of  the  paddle  A  B  that  is  efficient  in  propelling 
the  vessel,  and  the  line  A  D  represents  that  portion 
of  the  paddle  that  tends  to  lift  the  vessel  out  of  the 
water,  which,  consequently,  as  it  produces  no  propul- 
sive effect,  must  be  entirely  lost.  But  the  line  A  B, 
being  the  breadth  of  the  paddle,  we  will  suppose  rep- 
resents the  pressure  it  exerts  on  the  water,  which, 


104  TIIEOEY    OF   THE   PADDLE    WHEEL. 

according  to  the  resolution  of  forces,  is  divided  into 
two  other  pressures.  A  D,  tending  to  lift  the  vessel, 
is  the  useless  pressure,  and  B  D,  at  right  angles  to  the 
vessel's  path,  is  the  efficient  pressure,  or  the  portion 
that  is  utilized  in  propelling  the  vessel.  Power,  how- 
ever, is  not  composed  of  pressure  alone,  but  is  com- 
pounded of  pressure  and  velocity,  and  as  the  velocities 
of  the  columns  of  water,  having  A  D  B  D  for  the  base 
depend  upon  the  lengths  of  those  lines  respectively ; 
that  is  to  say,  if  we  double  the  length  of  either  one 
of  them,  say  B  D,  for  instance,  diminishing  the  angle 
at  C,  we  not  only  double  the  quantity  of  water  dis- 
placed in  any  given  time,  but  it  is  also  displaced  with 
double  the  velocity ;  the  power,  therefore,  developed 
is  the  product  of  these  two,  or  as  the  square.  Hence, 
it  follows  that,  since  A  D  represents  the  useless  pres- 
sure, the  square  of  that  line  must  represent  the  useless 
or  lost  power ;  or,  more  correctly,  the  loss  of  useful 
effect,  and  the  square  of  B  D,  the  power  that  is  applied 
to  propelling  the  vessel.  Now,  then,  considering  A  B 
to  be  unity,  the  square  of  B  D  will  be  the  square  of  the 
natural  sine  of  the  angle  BAD,  and  the  square  of 
A  D  the  square  of  the  natural  sine  of  the  angle  A  B  D ; 
but  the  triangles  A  B  D,  A  C  D',  being  similar,  the 
angles  at  B  and  C  are  equal,  and  the  loss  of  effect  is, 
therefore,  simply  represented  by  the  square  of  the  sine 
of  the  angle  that  the  oblique  arm  makes  with  the  per- 
pendicular; but  as  the  angle  is  continually  changing, 
as  the  arm  moves  through  the  water,  we  have  to  take 
the  mean,  and  the  more  numerous,  therefore,  the  divi- 
sions are  made,  the  nearer  correct  will  be  the  result. 

Thus,  supposing,  as  per  figure  2,  a  wheel  26  feet 
diameter,  from  outside  to  outside  of  paddles,  6  feet 
immersion  of  lower  edge  of  paddles,  and  20  inches 


THEORY  OF  THE  PADDLE  WHEEL. 


105 


breadth  of  paddles,  the   loss  from  oblique  action  is 
calculated  as  follows,  the  arc  being  divided  into  divi- 


Fio.  2. 


sions  of  5°  each,  which  are  considered  sufficiently  nu- 
merous for  practical  purposes : 


106 


THEOEY    OF   THE   PADDLE    WHEEL. 


• 

Angles 
of 
Incidence. 

Sines  of  the 
Angles  of 
Incidence. 

* 

55° 

.81915 

.33550 

=  half  of  the  square  of  sine 

1. 

50° 

.76604 

.58681 

=  square  of  sine. 

1. 

45° 

.10111 

.50000 

M 

1. 

40° 

.64279 

.41317 

<( 

1. 

35° 

.57358 

.32899 

<l 

1. 

30° 

.50000 

.25000 

—       u 

1. 

25° 

.42262 

.17860 

—       « 

1. 

20° 

.84202 

.11697 

—       n 

1. 

15° 

.25882 

.06698 

—       " 

1. 

10° 

.17365 

.03015 

—       u 

1. 

5° 

.08716 

.00759 

—       (( 

1. 

0° 

.00000 

.00000 

—       n 

I. 

5° 

.08716 

.00759 

—       (I 

1. 

10° 

.17365 

.03015 

—       it 

1. 

15° 

.25882 

.06698 

—       « 

1. 

20° 

.34202 

.11697 

—       <( 

1. 

25° 

.42262 

.17860 

U 

1. 

30° 

.50000 

.25000 

—         U 

1. 

35° 

.57358 

.32899 

—      It 

1. 

40° 

.62279 

.41317 

u 

1. 

45° 

.70711 

.50000 

—       it 

1. 

60° 

.76604 

.58681 

u 

i 

65° 

.81915 

.33550 

=  half  of  the  square  of  sine. 

22 

5.62952 

As  22  :  5.62952  :  :  100  :  25.588  per  cent,  of  the 
power  applied  to  the  wheels. 

Half  of  the  square  of  the  sine  at  the  angle  of  55°  is 
taken,  because  the  paddle  in  that  position  is  only  half 
immersed,  consequently  only  half  the  power  can  be 
expended  on  it  as  if  entirely  immersed  ;  and  the  angles 
are  put  down  twice,  because  the  loss  is  the  same  after 
the  paddle  leaves  the  vertical  position  as  before  it 
reaches  it.  The  power  in  the  latter  case  being  ex- 
pended in  forcing  the  water  downwards,  and  in  the 
former  case  in  lifting  the  water,  neither  of  which  as- 
sists in  propelling  the  vessel,  the  only  tendency  being 
to  lift  the  bow,  and  depress  the  stern. 

Slip. 

The  loss  of  effect  from  slip  is  usually  considered 
the  difference  between  the  velocity  of  the  centre  of 
pressure  of  the  paddles  and  the  velocity  of  the  vessel. 


THEORY  OF  THE  PADDLE  WHEEL.        10 7 

Thus,  if  the  velocity  of  the  centre  of  pressure  of 
the  paddles  exceeds  the  velocity  of  the  vessel  by  18 
per  cent,  of  the  speed  of  the  paddles,  18  per  cent,  is 
considered  the  loss  of  effect  from  slip.  This  we  con- 
ceive to  be  an  error.  The  18  per  cent,  is  the  difference 
between  the  velocity  of  the  paddles  and  the  velocity 
of  the  vessel,  nothing  more ;  and,  therefore,  simply 
represents  the  slip  in  per  cent,  of  the  paddles,  but  not 
the  loss  of  effect  from  slip.  For  it  has  been  shown 
that  the  loss  resulting  from  the  oblique  action  of  the 
paddles  on  the  water,  is  as  the  squares  of  the  sines  of 
the  angles  of  incidence,  and  if  we  suppose  the  wheel 
to  be  immersed  to  its  axis,  the  loss  from  this  cause  on 
the  paddle,  when  in  the  horizontal  position — the  angle 
being  90° — is  100  per  cent.,  and  if  the  loss  from  slip 
of  18  per  cent,  be  added  to  that,  we  have  a  total  loss 
of  118  per  cent.,  or  more  than  the  power  applied.  A 
positive  absurdity.  Or,  again,  supposing  the  vessel  to 
be  made  fast  to  the  wharf,  the  difference  between  the 
velocity  of  the  paddles  and  the  velocity  of  the  vessel 
will  be  100  per  cent.,  and  as  the  loss  from  oblique  ac- 
tion cannot,  from  this  circumstance,  be  any  less  than 
if  the  vessel  was  moving  ahead,  there  will  be  a  total 
loss  of  the  power  applied  to  the  wheels  of  125.588  per 
cent.  A  result  equally  absurd. 

At  the  angle  of  45°  it  has  been  seen  that  only 
.70711  part  of  the  area  of  the  paddle  is  effective  in 
propelling  the  vessel,  and  that  at  this  angle  the  ve- 
locity of  the  column  of  water  driven  aft  is  only  .70711 
of  what  it  is  when  the  whole  area  of  the  paddle  is 
effective,  hence  the  power  expended  in  slip  =  .70711 
X  .70711  =  5,  the  slip  in  the  vertical  position  being 
considered  1. 

Now,  then,  if  18  per  cent,  is  the  loss  from  slip 


108  THEOKY   OF   THE   PADDLE   WHEEL. 

when  the  paddle  is  in  the  vertical  position — which 
must  be  the  case  if  its  velocity  exceeds  that  of  the 
vessel  by  18  per  cent,  of  its  own  speed — from  what 
has  just  been  shown,  at  the  angle  of  45°,  the  loss  can- 
not be  more  than  half  of  18,  or  9  per  cent.  The  same 
reasoning  will  demonstrate,  that  at  the  angle  of  30° 
the  loss  from  slip  cannot  exceed  f  of  18,  or  13.5  per 
cent.  Thus  we  see  the  loss  from  slip  goes  on  decreas- 
ing from  the  vertical  to  the  horizontal  position,  at 
which  place  it  becomes  nothing.  We  can,  therefore, 
approximate  very  nearly  to  the  true  loss  in  the  present 
radial  wheel,  by  taking  the  mean  of  these  losses  at  the 
angles  as  laid  down  in  figure  2.  They  are  as  follows: 

At   0°  =  18  —      .0000  =  18       _  per  cent. 

"  5°  =  18  -      .1366  =  17.8634  "  " 

"  10°  =  18  —      .5427  =  17.4573  "  " 

"  15°  =  18  —    1.2056  =  16.7944  "  " 

"  20°  =  18  -    2.1055  =  15.8945  «  " 

"  25°  =  18  -    3.2148  =  14.7852  "  " 

"  30°  r=  18  -    4.5000  =  13.5000  "  " 

'   "  35°  =  18  -     5.9218  =  12.0782  "  " 

"  40°  =  18  -    7.4371  =  10.5629  "  " 

"  45°  =  18  —    9.0000  =    9.0000  "  " 

"  50°  =  18  -  10.5626  =    7.4374  "  " 

"  55°  =  18  —  12.078    =    5.9220  "  " 
~~2~~              138.3343 


Doubled  for  both  sides  of  the  vertical )  t)  7  /  •  n  /  •  o  /  • 

position  \    •     "    Ai  O.DDOO 

18.0000 

294.6686 

294.6686 


=  13.394  per  cent,  of  the  power  applied  to 


the  wheels. 


THEOEY  OF  THE  PADDLE  WHEEL.        109 

The  same  result  is  obtained  as  follows : 

100.000  (power  applied)  —  25.588  (oblique  action) 

X  18  per  cent,  (slip  of  the  vertical  paddle)  =  13.394 

per  cent. 

We  have,  therefore,  for  a  total  loss  in  this  radial 

wheel,  25.588  +  13.394  =  38.982  per  cent,  of  the  power 

applied  to  it. 

Feathering   Wheel. 

Let  us  take  a  feathering  wheel,  of  the  same  di- 
ameter of  centre  of  pressure,  i.  e.,  26  feet  4  inches  in 
diameter  from  outside  to  outside  of  paddles — same 
immersion,  breadth,  and  number  of  paddles,  and  see 
how  it  compares  with  this. 

It  is  conceived  by  some  that  the  only  losses  in  this 
kind  of  wheel  are  the  friction  of  the  eccentrics,  <fec., 
and  the  slip,  but  there  is  another  loss  with  deep  im- 
mersions, or  light  slips,  occasioned  by  the  drag  of  the 
paddles  as  they  enter  and  leave  the  water. 

In  figure  3,  the  paddles  are  supposed  to  be  verti- 
cal from  the  time  they  enter  until  they  leave  the 
water,  and  the  positions  of  the  arms  will  be  seen  at 
the  degrees  there  laid  down.  The  perpendicular  lines 
drawn  across  the  arcs  are  intended  to  represent  the 
breadth  of  the  paddles.  It  is  plain  that  while  the  axis 
of  tjie  paddle  moves  from  A  to  B,  it  moves  horizon- 
tally the  distance  A  C,  and  vertically  the  distance 
C  B,  and,  supposing  the  vessel  to  be  moving  with  the 
same  velocity  as  the  paddles,  it  will  travel  the  distance 
A  B,  while  the  paddle  travels  horizontally  the  distance 
A  C.  Now,  the  distance  A  C  being  less  than  A  B, 
the  paddle  in  this  position  cannot  be  giving  out  any 
power,  but  must  be  keeping  the  vessel  back,  by  carry- 


110 


THEORY    OF   THE   PADDLE    WHEEL. 


ing  a  column  of  water  before  it,  the  base  of  which  is 
equal  to  the  area  of  the  paddles,  and  the  length  equal 
to  the  difference  in  the  lengths  of  the  two  lines. 


FIG.  a 


If  A  B  be  represented  by  unity,  A  C  will  be  rep- 
resented by  the  natural  sine  of  the  angle  ABC,  and 
if  the  arc  be  supposed  to  be  divided  into  an  infinite 


THEOEY  OF  THE  PADDLE  WHEEL.        Ill 

number  of  parts,  or  composed  of  an  infinite  number  of 
straight  lines,  A  B  will  be  at  right  angles  to  A  D,  and,  by 
consequence,  the  angle  ABC  will  be  equal  to  the  angle 
DAE;  and  as  the  sine  of  B  represents  the  distance 
traveled  horizontally  by  the  paddle,  the  sine  of  D  A  E 
must  manifestly  represent  the  same  thing,  but  the  sine 
of  D  A  E  is  the  cosine  of  D,  which  therefore  repre- 
sents the  horizontal  velocity  of  the  paddle  at  the  angle 
of  50°,  its  circular  velocity  being  1.  The  difference 
between  these  two  lines  is,  therefore,  the  loss  from 
'li'acj,  supposing  there  to  be  no  slip,  but  as  all  paddle 
wheels  must  have  some  slip,  when  they  are  propelling 
a  vessel,  the  line  A  B,  diminished  by  the  amount  of 
slip,  will  represent  the  distance  traveled  by  the  vessel, 
and  the  loss  from  drag  will  therefore,  instead  of  being 
the  difference  between  A  B  and  A  C,  be  the  difference 
between  a  fraction  of  A  B  and  the  whole  of  A  C,  de- 
pendent upon  the  amount  of  slip.  If  this  fraction  of 
A  B  be  just  equal  to  A  C,  the  loss  from  drag  in  this 
position  becomes  0 ;  for,  though  the  paddle  be  giving 
out  no  power  to  the  vessel,  it  occasions  no  resistance 
to  the  vessel's  progress  through  the  water,  because  it 
is  moving  horizontally  precisely  as  fast  as  the  vessel 
itself;  and  if  the  fraction  be  less  than  A  C,  the  resist- 
ance will,  of  course,  be  on  the  after  instead  of  the  for- 
ward side  of  the  paddle,  and  it  must,  in  consequence, 
necessarily  be  assisting  in  propelling  the  vessel. 

Now,  then,  from  the  above,  it  must  be  evident  to 
any  one,  that  so  long  as  the  paddle,  after  it  enters  the 
water,  is  moving  horizontally  at  a  less  rate  than  the 
vessel,  it  cannot  be  giving  out  any  power,  but  must  be 
an  actual  resistance  to  the  vessel's  progress  through 
the  water.  Taking  figure  3,  and  giving  the  wheel  the 

same  mean  loss  from  slip  as  the  radial  wheel,  viz., 
8 


112  THEOEY    OF   THE   PADDLE    WHEEL. 

13.394  per  cent.,  we  will  ascertain  the  loss  from  slip 
at  the  different  angles  there  laid  down,  and  attend  to 
the  drag  afterwards,  which  is  merely  slip  in  the  oppo- 
site direction,  or  what  might  be  termed  negative  slip. 

To  give  this  wheel  the  same  mean  loss  from  slip  as 
the  radial  wheel,  it  has  to  have  on  the  arm  when  in 
the  vertical  position,  or 

At  0°       CoBlne,                         26.225  per  cent. 

"     5°  =  .99619  -  .73775  =  25.844  "  " 

"  10°  =  .98481  -  .73775  =  24.706  "  " 

"  15°  =  .96593  -  .73775  =  22.818  "  " 

"  20°  =  .93969  -  .73775  =  20.194  "  " 

"  25°  =  .90631  -  .73775  -  16.856  "  " 

"  30°  =  .86603  -  .73775  =  12.828  "  u 

"  35°  =  .81915  -  .73775  =    8.140  "  " 

"  40°  -  .76604  -  .73775  =    2.829  "  " 

"  45°  =                                     0.000  "  " 

"  50°=                                      0.000  "  « 

"  55°=                                       0.000  "  " 

134.215 


Doubled  for  both  sides  of  the  vertical  P 
position,  5 

26.225 


294.655 

994  655 

-  =  13.394  per  cent,  of  the  power  applied  to 
2i2 

the  wheel  lost  by  slip. 

At  the  angle  of  55°  the  paddle  is  .445  part  im- 
mersed, but,  being  so  near,  we  have  taken  it  at  a  half 
for  simplicity,  and  for  like  reason  have  considered  the 
paddle  at  50°  entirely  immersed. 

It  will  be  seen  from  the  above,  that  the  paddle, 
from  the  time  it  enters  the  water  until  after  it  passes 


THEORY  OF  THE  PADDLE  WHEEL.        113 

45°,  is  traveling  horizontally  at  a  less  rate  than  the 
vessel,  and  the  same  effect  ensues  as  it  rises  out  of  the 
water ;  there  must,  therefore,  be  a  loss  from  drag  or 
negative  slip.  Let  us  see  what  this  amounts  to. 

Cosines. 

.73775  -  .57358 
At  55°  =  -    -g-     -  8.208  per  cent. 

«  50°  =  .73775  -  .64279  =  9.496  "   " 
"  45°  =  .73775  -  .70711  =  3.064  "  . " 


20.768 
2 


Doubled  for  entering  and  leaving,  41.536 

41.536 

— OS —  =  1.933  per  cent. 

We  have,  then,  for  a  total  loss  in  this  wheel,  slip 
(13.394  per  cent.)  -f  drag  (1.933  per  cent.)  =  15.327 
per  cent,  of  the  power  applied  to  it. 

The  total  loss  in  the  radial  wheel  having  been 
shown  to  be  38.982  per  cent,  (and  in  the  feathering 
wheel  15.327  per  cent.),  we  have  23.655  per  cent, 
in  favor  of  the  feathering  wheel.  But  of  the  whole 
power  applied  to  the  engines,  about  20  per  cent,  is  ex- 
pended in  overcoming  friction  of  ditto,  friction  of  load 
on  working  journals,  working  air  and  feed  pumps  with 
their  loads,  &c.  Consequently,  only  80  per  cent, 
reaches  the  wheels,  and  23.655  per  cent,  of  80  per 
cent,  equals  18.924  per  cent,  of  the  total  power  applied 
to  the  engines  in  favor  of  the  feathering  wheel. 

To  stand  off  against  this,  we  have  the  friction  of 
the  eccentrics,  &e.  (an  amount  that,  perhaps,  can  only 
be  estimated)  extra  weight  and  wear  and  tear  of  the 
wheels. 

It  will  be  seen  also  from  the  above,  that  the  differ- 
ence between  the  velocity  of  the  feathering  wheel  and 


114  CENTRE    OF    PKESSUEE. 

the  vessel  being  26.235  per  cent,  of  the  speed  of  the 
wheel,  and  the  difference  between  the  velocity  of  the 
radial  wheel  and  the  vessel  being  18  per  cent,  of  its 
speed,  it  follows  that,  making  the  same  number  of 
revolutions,  the  speeds  of  the  vessels  will  be  as  73.775 
to  82,  or  as  1.00  to  1.11 ;  consequently,  the  speed  of 
the  feathering  wheel  will  have  to  exceed  the  speed 
of  the  radial  wheel  11  per  cent,  to  give  the  vessel  the 
same  velocity,  but  this  speed  of  the  wheel  is  as  shown— 
consequent  upon  there  being  less  resistance  to  the  pad- 
dles— attained  by  an  expenditure  of  18.924  per  cent. 
less  power. 

Centre  of  Pressure. 

The  centre  of  pressure  of  a  rectangular  plane  im- 
mersed in  a  fluid,  the  upper  extremity  of  which  is  even 
with  the  surface  of  the  fluid,  is  ^  from  the  bottom ; 
but,  inasmuch  as  the  pressure  is  as  the  depth,  when  its 
upper  extremity  is  below  the  surface  of  the  fluid,  this 
law  no  longer  holds  good.  To  ascertain  the  centre  of 
pressure  in  such  case,  "  Jamieson  on  Fluids  "  gives  the 
following  practical  rule  deduced  from  elaborate  math- 
ematical calculations : 

"  Divide  the  difference  ot  the  cubes  of  the  extremi- 
ties of  the  given  plane  below  the  surface  of  the  fluid, 
by  the  difference  of  their  squares,  and  two-thirds  of 
the  quotient  will  give  the  distance  of  the  centre  of 
pressure  below  the  surface,  from  which  subtract  the 
depth  of  the  upper  extremity,  and  the  remainder  will 
show  the  point  in  the  centre  line  of  the  plane  in  which 
the  centre  of  pressure  is  situated." 

This  rule  can  be  applied  directly  to  the  feathering 
wheel,  by  taking  the  mean  immersion  of  the  paddles 


CENTRE   OF   PRESSURE.  115 

as  they  move  through  the  water,  and  assuming  figure  3 
to  be  of  the  same  diameter  from  outside  to  outside  of 
paddles,  as  figure  2,  viz  :  26  feet,  we  find  the  mean 
immersion  of  the  lower  edges  of  the  paddles,  after  their 
upper  extremity  gets  below  the  surface,  to  be 

(29.23  +  37.84+45.59  -f-  52.44  -f  58.32  -f-  63.09  +  67.02  -f-  69.78  +  71.44) 
19 

2  -j-  72  =  55.87  inches,  and  upper  edge  35.87  inches. 
The  mean  centre  of  pressure  of  the  paddles  in  these 

/  55.87"—  35.  87s  \ 
positions  is  (558r_3587ij|  =  46.59  —  35.8  7  =  10.72 

inches  from  top,  or  9.28  inches  from  bottom,  and  the 
mean  centre  of  pressure  from  the  time  the  paddle 
enters  until  it  leaves  the  water, 


In  the  radial  wheel,  however,  as  the  outer  ex- 
tremity of  the  paddle  moves  more  rapidly  than  the 
inner  extremity,  and  as  the  resistance  is  as  the  square 
of  the  velocity,  the  centre  of  pressure  must  be  consid- 
erably nearer  the  outer  extremity  on  this  account. 
One-third  from  the  bottom,  in  this  case,  is,  therefore, 
probably,  not  much  out  of  the  truth  ;  but  as  a  portion 
of  the  paddle  only  part  of  the  time  is  immersed,  we 
take  the  mean  of  the  third  of  that  portion  and  a  third 
of  the  whole  breadth  of  the  paddle  during  the  time  it 
is  entirely  immersed. 


TV.  --  -- 

Thus  :  v  ~^f-  =  b.tf  inches  from 

2o 

,  the  bottom,  showing  the  centre  of  pressure  under  these 
circumstances  to  be  (8.52  —  6.37  =  )  2.15  inches  nearer 
the  lower  edge  of  the  paddle  in  the  radial  than  it  is  in 
the  feathering  wheel. 


116  THE   SCREW   PROPELLER. 

Practical  Remarks  on  tlie  Foregoing. 

From  what  has  been  shown,  it  would  appear  that 
the  use  of  the  feathering  wheel  over  the  radial  wheel, 
from  the  great  saving  it  effects,  would  lead  to  its  uni- 
versal adoption ;  but,  unfortunately,  the  practical  diffi- 
culties are  such  that  its  use  is  confined  within  very 
narrow  limits.  The  increased  weight  of  the  wheel, 
occasioned  by  the  eccentrics,  levers,  arms,  <fec.,  required 
to  work  the  paddles,  amounting,  in  some  cases,  to 
several  tons,  causing  the  pillow-block  brasses  to  wear 
away  very  rapidly,  is  a  sad  objection,  to  say  nothing 
of  the  excessive  friction  they  produce.  Besides,  the 
pins  operating  as  the  axis  about  which  the  paddles 
vibrate  are  found  to  wear  away  very  rapidly,  requiring 
not  only  to  be  replaced  frequently,  but  the  noise  and 
jar  occasioned  from  the  wear  becomes  very  objec- 
tionable. The  latter  objection,  however,  can  be  re- 
moved by  the  use  of  lignumvitse  pin  bearings. 

The  Screw  Propeller. 

The  great  advantages  derivable  from  the  successful 
adaptation  of  the  screw  propeller,  particularly  to  ves- 
sels of  war,  became  well  understood  in  its  early  his- 
tory, and  inventive  genius  set  to  work  thenceforth  to 
perfect  this  important  invention ;  all  kinds  of  propel- 
lers sprang  into  use,  many  of  them  possessing  neither 
the  merit  of  novelty  nor  usefulness.  One,  two,  three, 
four,  five,  six-bladed,  true  screws,  expanding  pitch  and 
no  screw  at  all,  are  among  the  number  that  have  been 
tried  experimentally  and  practically  since  the  intro- 
duction of  the  screw  propeller,  and,  strange  as  it  may 
appear,  notwithstanding  the  large  share  of  attention  it 
has  received,  the  theory  of  the  screw  propeller  is  yet  not 


THE   SCREW    PROPELLER.  117 

generally  understood;  but,  to  our  mind,  this. is  owing 
to  one  great  cause ;  and  that  is,  to  the  very  important 
fact,  that  those  who  have  undertaken  to  explain  and 
illustrate  it,  have  apparently  thought  it  more  impor- 
tant to  give  the  history  and  accounts  of  the  experi- 
ments— though  both  very  useful  in  themselves — than 
to  explain  the  leading  features  and  the  laws  governing 
its  action.  Besides,  a  practical  engineer  does  not  wish, 
or  if  he  did,  has  not  the  time  to  spare,  to  examine 
large  volumes  to  find  what  might  be  condensed  into  a 
few  pages.  We  have,  therefore,  determined  to  make 
our  remarks  on  this  subject  brief,  and  to  confine  them 
to  those  points  which  we  think  are  the  more  impor- 
tant, allowing  the  student  to  build  upon  them  for  him- 
self. 

The  surface  of  a  screw  blade  may  be  supposed  to 
be  generated  by  a  line  revolving  around  a  cylinder,  at 
right  angles  to  the  axis,  at  the  same  time  that  it  moves 
along  it,  and  should  the  revolving  motion  be  a  constant 
ratio  to  the  motion  lengthwise,  it  will  be  a  true  screw. 
Should  such  a  screw  as  this,  Fig.  4,  be  developed  upon 
a  plane  it  will  form  a  FIG.  4. 

right-angled  triangle,  in 
which  A  B  is  the  pitch, 
A  C  the  circumference 
described  by  the  extrem- 
ity of  the  blade,  and  B  C 
the  line  described  by  any 
point  in  the  periphery 
of  the  blade  by  one  con- 
volution of  the  thread.  To  make  this  the  more  clear, 
suppose  the  triangle  A  B  C  to  be  wound  round  a  cyl- 
inder, having  a  circumference  equal  to  A  C,  and  sup- 
pose at  C  we  start  to  trace  a  line  around  the  cylinder, 


118  THE   SCREW   PEOPELLEE. 

moving  along  it  at  the  same  time  in  a  constant  ratio. 
and  that  when  we  have  gone  all  the  way  around,  ar- 
riving over  the  starting  point  C,  (C  and  A  will  be  one 
and  the  same  point  in  the  case  supposed)  we  have 
reached  the  point  B,  C  B  will  be  the  line  described, 
which  is  technically  termed  the  directrix,  and  A  B, 
being  the  distance  moved  in  the  direction  of  the  axis, 
will  be  the  pitch.  Should  the  line  A  B  be  a  curve, 
instead  of  a  straight  line,  the  screw  would  have  an  in- 
creasing or  expanding  pitch,  instead  of  an  uniform 
pitch.  Figure  5  will  illustrate  this:  Let  the  curve 
FIG.  5.  B  C  be  the  curve  of  the  blade, 

and  the  dotted  lines  B  £,  C  c 
be  tangents  drawn  to  this 
curve,  it  will  be  seen  that,  at 
different  points  in  the  curve 
B  C,  the  velocity  of  rotation 
remaining  constant,  the  ve- 
locity lengthwise  of  the  axis 
A  B  varies,  growing  greater 
as  we  approach  B.  This  is  what  is  termed  an  expand- 
ing pitch ;  that  is  to  say,  the  pitch  at  the  anterior  por- 
tion of  the  blade,  is  less  than  the  pitch  at  the  posterior 
portion.  The  object  of  such  a  pitch  is  this :  the  ante- 
rior portion  of  the  blade  striking  upon  water  at  rest, 
encounters  the  resistance  due  to  a  solid  body  moving 
through  water  at  rest,  but  this  portion  of  the  blade 
puts  the  water  in  motion,  it  being  a  yielding  medium, 
so  that  when  the  posterior  portion  of  the  blade  follows 
it  has  to  act  on  water  in  motion,  instead  of  water  at 
rest,  and  in  order,  therefore,  to  make  the  resistance 
due  to  all  parts  of  the  blade  alike,  the  pitch  of  the  pos- 
terior portion  of  the  blade  is  increased  to  the  extent 
of  the  motion  given  to  the  water  by  the  anterior  por- 
tion. 


TILE   SCKEW    PROPELLEK.  119 

To  measure  the  pitch  of  a  screw  blade,  did  it  ex- 
tend all  the  way  round  the  shaft  to  a  full  convolution 
of  the  thread,  all  we  would  have  to  do,  would  be  to 
measure  along  the  line  of  the  shaft  from  any  point  in 
the  blade  to  any  point  directly  over  it,  and  the  dis- 
tance would  be  the  pitch,  or  the  distance  traveled  in 
the  direction  of  the  axis  by  one  convolution  of  the 
thread ;  but  since  in  practice,  in  order  to  secure  the 
proper  resisting  area,  a  full  convolution  of  the  thread 
is  not  required — a  very  small  fraction  of  it  being 
used — it  becomes  necessary,  therefore,  to  find  the  pitch 
from  this  fraction.  Taking  figure  4,  for  instance,  let 
B  b  be  the  length  of  the  blade,  measured  on  the  peri- 
phery, and  A  C  the  circumference  described  by  the 
extremity  of  the  blade,  B  b  will  be  the  fraction  of  the 
blade  used,  and  B  a  the  fraction  of  the  pitch.  We 
know,  therefore,  that,  starting  from  B,  and  traveling 
along  the  line  B  £,  when  we  arrive  at  the  point  $,  we 
have  traveled  along  the  axis  the  distance  B  #,  and  from 
this  we  can  ascertain  what  distance  will  be  moved 
along  the  axis  by  continuing  all  the  way  round  until 
we  arrive  at  C,  which  will  be  the  pitch.  Practically, 
we  can  measure  this  in  two  ways :  measure  the  length 
B  b  of  the  blade,  and  also  B  «,  the  length  in  line  with 
the  axis,  we  have  then  two  legs  of  a  right-angled  tri- 
angle, from  which  we  ascertain  the  third,  a  b.  Now, 
then,  knowing  the  circumference  described  by  the  ex- 
tremity of  the  blade,  we  derive  the  following  simple 
proportion : 

As  a  b  :  the  whole  circumference  :  :  B  a  :  the  whole 
pitch. 

Or  we  proceed  thus :  Lay  a  straight-edge  across 
the  face  of  the  propeller,  at  right  angles  to  the  axis, 
and  a  bevel  on  the  periphery  of  the  blade,  and  look 


120  THE   SCREW    PEOPELLEE. 

them  out  of  wind,  the  angle  enclosed  by  the  two  legs 
of  the  bevel  will  be  the  angle  B  b  a,  which  is  termed 
the  "  angle  of  the  propeller ; "  and  hence,  if  B  b  be 
supposed  unity,  the  fraction  of  the  pitch  of  the  one 
blade  will  be  (B  a)  the  natural  sine  of  the  angle  B  b  a, 
therefore,  knowing  the  angle  B  b  a,  and  the  length  of 
the  blade  B  3,  we  ascertain  the  pitch  thus : 

As  cosine  b  :  whole  circumference  of  propeller  :  : 
sine  b  :  to  whole  pitch. 

The  pitch  can  also  be  determined  by  construction, 
without  any  calculation  whatever.  Thus,  supposing 
the  line  a  b  represents  the  whole  circumference  of  the 
propeller,  we  draw  the  line  B  b  at  the  angle  to  a  b  as- 
certained from  measurement,  and  erect  the  perpendic- 
ular a  B,  which  will  give  the  pitch  required. 

.  In  a  true  screw,  it  matters  not  whether  we  take 
the  angle  at  the  periphery  or  any  other  part  of  the 
blade  ;  for,  though  the  angle  will  be  different,  increas- 
ing as  we  approach  the  centre,  the  pitch  will  be  the 
same,  it  only  being  necessary  to  know  the  circumfer- 
ence at  the  point  where  we  measure  the  angle. 

Should  the  blade  not  be  a  true  screw,  but  an  ex- 
panding pitch,  we  have  to  take  the  angle  at  two  or 
more  points,  by  drawing  tangents  to  the  curve,  and 
take  the  mean,  for  the  mean  angle  of  the  blade.  Thus, 
in  figure  5,  the  mean  of  the  angles  B  b  A  and  c  C  A 
will  give  the  mean  angle  of  the  blade. 

Some  propellers  are  made  to  expand  from  hub  to 
periphery,  instead  of  from  anterior  to  posterior  portion 
of  the  blade. 

To  ascertain  the  pitch  of  such  a  propeller,  take  the 
mean  of  the  angles  at  several  points  in  the  blade,  and 
proceed  as  above.  In  order  to  ascertain  the  pitch  of 
any  propeller,  it  is  always  proper  to  take  the  angles  at 


THE    SCREW   PEOPELLER.  121 

two  or  more  points  in  the  blade,  from  which  we  learn 
whether  it  expands  from  hub  to  periphery,  whether  it 
be  true  screw,  or  no  screw  at  all. 

The  fraction  of  the  pitch,  as  we  have  explained  it 
above,  is  the  fraction  of  the  pitch  of  one  blade,  but  as 
screw  propellers  usually  have  two,  three,  four,  six,  <fec., 
blades,  constituting  fractions  of  a  double-threaded, 
treble-threaded,  four-threaded,  six-threaded,  <fec.,  screw, 
the  sum  of  these  constitute  the  fraction  of  what  is 
usually  termed  the  fraction  of  the  pitch  of  the  screw ; 
that  is  to  say,  if  the  screw  have  three  blades,  and  the 
fraction  of  the  pitch  of  one  of  those  blades  be  T'^-,  the 
real  fraction  of  the  pitch  will  be  3  times  TV,  or  £ ; 
for  it  evidently  matters  not,  as  far  as  this  is  concerned, 
whether  the  screw  be  in  one,  or  divided  into  a  dozen 
parts. 

How  to  lay  down  a  Propeller. 

Knowing  the  diameter,  number  of  blades,  and  frac- 
tion of  pitch,  we  intend  to  use,  we  proceed  thus  : 

Taking  figure  6,  for  in- 
stance, draw  the  line  A  C, 
equal  the  circumference  of 
the  extremities  of  the  blades, 
and  from  A  erect  the  per- 
pendicular A  B,  equal  the 
pitch ;  join  B  C.  Now,  then, 
supposing  we  desire  the  pro-  A 
peller  to  have  four  blades,  and  vthe  fraction  of  the  pitch 
to  be  ±,  lay  off  B  a,  equal  to  TV  B  A,  and  draw  a  c, 
parallel  to  AC.  a  c  will  be  the  circumference  of  the 
extremity  of  one  blade  viewed  as  a  disc.  Then,  taking 
figure  T,  we  describe  the  circle  a  l>  c,  equal  A  C,  figure 


122  THE   SCREW   PEOPELLER. 

6,  and  also  the  smaller  circle,  equal  the  circumference 
of  the  hub  of  the  propeller  ;  divide  the 
larger  circle1  into  four  equal  parts,  and, 
from  the  centres  thus  obtained  lay  off 
a  d,  h  i,  bf,  e  c,  each  equal  to  a  e,  fig- 
ure 6,  and  draw  lines  from  each  of  these 
points  to  the  centre,  terminating  in  the 
hub  ;  such  will  be  the  projection  of  a  four-bladed,  true 
screw  propeller,  viewed  from  the  stern,  from  which 
the  longitudinal  elevation  can  be  drawn.  The  dimen- 
sions of  the  sections  of  the  blade  depend  upon  the 
diameter  of  the  propeller,  the  material  of  which  it  is 
constructed,  and  the  pressure  it  has  to  sustain. 

Centre  of  Pressure. 

All  solid  bodies  moving  through  a  fluid  have  a  cer- 
tain point  called  the  centre  of  pressure,  which  is  the 
point  where  the  outer  and  inner  pressures  just  exactly 
balance.  In  a  screw  propeller,  the  radius  of  the  cir- 
cle, which  is  equal  to  half  the  area  of  the  whole  circle, 
described  by  the  periphery  of  the  blades,  is  the  centre 
of  pressure  from  centre  of  motion.  Thus,  if  a  propeller 
be  16  feet  diameter,  the  area  of  the  circle  described  by 
the  extremity  of  the  blades  =  201.06  square  feet,  and 
the  radius  of  the  circle,  having  an  area  equal  to  half 
this,  is  5  feet  T|  inches,  consequently  the  centre  of 
pressure  in  this  propeller  is  5  feet  Y^  inches  from  the 
centre  of  shaft. 

The  centre  of  pressure  can  also  be  ascertained  in 
the  following  manner : 

1  +4  +  9+16+25+36+49+64        204 

1 1 1 1—          —  I 1 1 1  = =  5  feet  8 

1  +2  +  3+  4+5  +  6  +  7  +  8 

inches,  nearly  as  before. 


THE   SCEEW   PROPELLEE.  123 

The  line  per  sketch  represents  the  radius  of  the 
propeller,  and  is  divided  into  divisions  of  1  foot  each ; 
the  more  numerous,  of  course,  the  divisions  are  made, 
the  nearer  correct  will  be  the  result. 

In  these  calculations,  the  area  of  the  hub  is 
neglected. 

The  above  rule  holds  good  so  long  as  there  is  no 
variation  in  the  pitch  from  hub  to  periphery ;  but 
should  the  pitch  vary  in  this  direction,  the  velocity  of 
the  column  of  water  driven  aft  from  different  parts  of 
the  blade  will  also  vary,  effecting  the  centre  of  pres- 
sure correspondingly. 

Slip. — The  slip  of  a  screw  propeller  is  the  differ- 
ence between  the  velocity  of  the  propeller  and  the 
velocity  of  the  ship. 

EXAMPLE. — A  propeller  having  20  feet  pitch  makes 
70  revolutions  per  minute,  which  propels  the  vessel  at 
the  rate  of  12  knots  an  hour,  required  the  slip,  the  sea 
knot  containing  6082|  feet? 

ANSWEE. 

20  X  70  X  60=84000=  speed  of  propeller  in  ft.  per  hour. 
6082|xl2=72992=         "     vessel         "        " 
11008=  slip  in  feet. 

84000  :  11008  :  :  100  :  13.1  =slip  in  per  cent,  of 
the  speed  of  the  propeller. 

Thrust. — A  propeller  being  put  in  revolution  throws 
a  column  of  water  off  from  the  blades  in  line  with  the 
axis  of  the  propeller,  which,  as  explained  above,  is  the 
slip;  the  resistance  of  this  water  acting  upon  the  pro- 
peller blades,  tends  to  force  the  shaft  inboard,  which 


124  THE   SCEEW   PEOPELLEE. 

resistance  has  to  be  sustained  by  heavy  bearings  called 
thrust  bearings,  and  the  amount  of  this  resisting  pres- 
sure is  called  the  thrust.  In  order,  in  practice,  to  as- 
certain the  extent  of  the  thrust,  an  instrument  called 
the  dynamometer  is  attached  to  some  part  of  the  shaft. 
This  instrument  consists  of  a  combination  of  levers  or 
weighing  beams,  to  the  final  end  of  which  is  attached 
a  spring  balance,  or  scale,  which  indicates  the  pressure 
in  pounds ;  and  this  pressure  being  augmented  by  the 
number  of  times  the  levers  are  multiplied,  gives  the 
total  pressure,  or  thrust  on  the  shaft.  And  the  total 
thrust  being  multiplied  into  the  distance  moved  over 
in  a  unit  of  time  by  the  vessel,  shows  the  actual  power 
absorbed  in  propelling  the  vessel. 

In  the  application  of  the  dynamometer,  care  must 
be  taken  that  it  receives  the  entire  thrust  of  the  shaft 
before  the  indication  of  the  scale  is  noted. 

Did  the  propeller  and  steam  piston  travel  through 
the  same  distance  in  any  given  time,  and  were  all  the 
power  applied  to  the  piston  transmitted  to  the  water 
through  the  propeller,  the  total  pressure  upon  the 
steam  piston  and  the  thrust  of  the  propeller  would  be 
identical,  but  since  such  is  never  the  case,  we  ascertain 
the  theoretical  thrust,  thus : 

Total  effective  pressure  on  piston  in  Ibs.  x  2  length  of  stroke  in  ft.  x  Xo.  of  revols.  per  inin. 
Pitch  of  propeller  in  feet  x  number  of  revolutions  per  minute. 

=  Theoretical  thrust  in  Ibs.  The  difference  between 
this  and  the  actual  thrust,  shows  the  amount  lost  in 
friction  of  engines,  propeller,  and  load,  overcoming 
resistance  to  edge  of  propeller  blades,  working  pumps, 
etc.  The  loss  from  slip  is  independent  of  this. 


THE   SCREW    PROPELLER. 


125 


Strain  upon  a  Screw  Propeller-blade. 

We  can  best  illustrate  this  by  an  example. 

Given,  circumference  of  centre  of  pressure  of  a  3 
bladed  propeller,  30.9  feet ;  distance  from  hub  to  cen- 
tre pressure  41  inches  ;  pitch  22.5  feet ;  thrust  12700 
pounds  :  required,  the  strain  upon  each  blade  at  the  hub. 


FIG 


SOLUTION. 

Let  F  G  H  be  the 

development  of  the  he- 

lix on  a  plane,  draw  B  D 

at  right  angles  to  F  H, 

and  A  E  at  right  angles 

to  Gr  H.    Trigonometri 

cally,  we  ascertain  the  c 

angles  at  A  and  D  to  be 

each  =  3*7°  9',  and  at  C  and  B  to  be  each  =  52°  5',  and 

the  lengths  of  the  lines  A  E,  B  D,  to  be  relatively  as 

1.000  to  1.237. 

Now,  inasmuch  as  the  whole  thrust  can  be  supposed 
to  be  concentrated  in  the  centre  of  pressure  of  the  blade, 
and  as  the  12700  Ibs.  is  in  a  line  with  the  axis,  it  follows 
that,  if  the  line  A  E  represents  the  direction  and  amount 
of  this  thrust,  the  line  B  D,  at  right  angles  to  the  pro- 
peller blade  at  the  centre  of  pressure,  according  to  the 
resolution  of  forces,  will  represent  the  resultant  of  the 
pressures  on  the  blade,  or  the  total  pressure  tending 
to  break  it.  But  inasmuch  as  there  are  three  blades, 
the  pressure  will  be  divided  equally  among  them  all  ; 
therefore,  each  has  to  sustain  but  a  third  of  this  pres- 
sure; hence 

12700  X  1.237  (proportion  B  D  bears  to  A  E)  __ 


Ibs.  pressure  on  each  blade  at  the  centre  of  pressure. 


1:20  THE    SCREW    PROPELLER. 

The  pressure  at  the  hub  on  each  blade  equals 
5236  Ibs.  X  41  ins.  =  21467G  Ibs.  acting  with  the 
leverage  of  one  inch. 

EXAMPLE  2o. — Suppose,  in  example  1,  the  breadth 
of  the  blades  at  the  hub  to  be  32  inches,  and  the  pro- 
peller to  be  made  of  composition,  capable  of  sustaining 
a  pressure  per  square  inch  of  cross-section  of  520  Ibs., 
acting  through  the  leverage  of  1  inch ;  required,  the 
mean  thickness  of  the  blade  at  the  hub  ? 

SOLUTION. — The  strength  of  beams  is  directly  as 
their  breadths  and  the  squares  of  their  depths,  and  in- 
versely as  their  lengths.  In  the  example  before  us, 
the  propeller  resolves  itself  into  a  simple  beam ;  we 

2146Y6X1       100.    ,  -,, 

nave,  then,  -  -  =  12.9  inches  =  square  01  the 

32  X  520 

thickness,  and  \/12.9  =  3.59  inches  in  thickness. 

Helicoidal  Area. — As  has  already  been  shown,  the 
development  of  the  helix  on  a  plane  is  the  hypothe- 
nuse  of  a  right-angled  triangle,  having  the  pitch  of  the 
screw  for  the  height;  and  the  circumference,  corre- 
sponding to  the  radii  of  the  helix,  for  the  base.  Now, 
as  the  propeller  can  be  supposed  to  have  an  infinite 
number  of  helices,  each  one  becoming  longer  and  longer 
as  we  approach  the  periphery,  which  alter  the  lengths 
at  the  same  time,  of  the  hypothenuse  and  base  of  the 
triangle,  we  will  suppose  the  propeller  to  be  divided 
into  a  number  of  concentric  rings,  taking  the  centre 
line  of  each,  for  the  helix  or  hypothenuse  of  the  trian- 
gle ;  the  circumference  corresponding  to  radii  of  said 
helix  for  the  base,  and  the  pitch  for  the  height,  from 
which  we  have  all  the  elements  required  for  the  cal- 
culation. 


THE   SCEEW    PROPELLER. 


127 


To  make  this  the  more  clear,  take   the  triangle 
B  A  C ;  the  lines  B  1,  B  2,  B  3,  B  C,  represent  the 


helices  having  the  corresponding  circumferences  of 
A  1,  A  2,  A  3,  and  A  C.  Now,  then,  if  these  helices 
he  the  lengths  of  the  rings,  or  elements  for  one  entire 
convolution  of  the  thread,  all  we  have  to  do  is  to  mul- 
tiply it  by  the  breadth  of  the  element,  which  will  give 
the  area  for  one  convolution ;  but  as  only  a  fraction 
of  a  convolution  is  used  in  practice,  we  multiply  by 
this  fraction,  whatever  it  may  be,  and  the  product 
gives  the  area  for  the  part  used.  This  mode  of  calcu- 
lation is,  of  course,  only  an  approximation ;  but  when- 
ever the  blade  is  divided  into  a  considerable  number 
of  elements,  say  6  inches  in  breadth  each,  the  result 
obtains  sufficiently  near  the  truth  for  all  practical  pur- 
poses. 

The  following  is  a  calculation  on  the  screw  of  the 
U.  S.  Steam  Frigate  "Wabash,"  and  which  agrees, 
within  a  very  small  fraction,  of  the  area  as  projected 
upon  a  plane: 


9 


128 


THE   SCEEW   PKOPELLER. 


Diameter  of  screw,  17  feet  4  inches  ;  diameter  of 
hub,  2  feet  4  inches. 


01 

?I 

"c 

f 

S 

«•••>  , 

«=! 

.    o"o    . 

^  K  =13 

o 

si 

H 

„  "a 

« 

®  rrj           03 

03  *S  .2  ^ 

O  on 

®  rt     . 

O    03 

d  2 

Pitch. 

N 

*3  0*0! 

5*^5 

ft 

55"* 

.si 

Sa 
i— 

Radii  o: 

^*  o 

|J<9 

w 

•£  0 

gs 

^  i 
MS 

Is 

01 

K 

o 

A 

B 

C 

D 

E 

F 

G 

H 

ft. 

ft. 

2.B  x  3.1416 
ft. 

VA2-j-C2 
ft. 

DxE 

ft. 

ft. 

FxG 
sqr.  feet. 

23 

1.5 

9.42 

24.89 

!/T 

7.11 

.5 

3.555 

2. 

12.56 

26.20 

" 

7.48 

3.74 

2.5 

15.70 

27.85 

It 

7.96 

3.98 

3. 

18.84 

29.73 

U 

8.49 

4.245 

3.5 

21.99 

31.82 

u 

9.09 

4.545 

4. 

25.13 

34.07 

II 

9.73 

4.865 

4.5 

28.27 

36.44 

(1 

10.41 

5.205 

5. 

31.41 

38.93 

(I 

11.12 

5.56 

5.5 

34.55 

41.50 

it 

11.86 

5.93 

6. 

37.69 

44.15 

14; 
/61 

12.12 

6.06 

6.5 

40.84 

46.87 

II 

12.86 

6.43 

7. 

43.98 

49.63 

3/ 
/ll 

13.54 

6.77 

7.5 

.    47.12 

52.43 

4/15 

13.78 

6.89 

8. 

50.27 

55.27 

V* 

13.82 

6.91 

8.5 

53.40 

58.14 

Vs 

11.63 

5.815 

*  HeliCoidal  area  of  one  side  of  both  blades  =  80.5  square  feet. 

Practical  Remarks  on  the  Screw  Propeller. 

In  the  application  of  power  to  the  propulsion  of 
the  hulls  of  vessels  through  water,  a  portion  of  the 
effect  is  lost  by  the  instrument  through  which  it  is 
transmitted.  In  the  common  radial  wheel  this  loss  of 
effect  is  compounded  of  two  losses,  slip,  plus  oblique 
action ;  in  the  feathering  wheel,  slip,  plus  drag,  and  in 
the  screw  propeller,  slip,  plus  friction  of  the  propeller 
blades  on  the  water.  That  instrument,  therefore,  which, 
possessing  no  more  practical  disadvantages  than  other 

*  For  the  calculations  of  the  friction  of  a  screw  surface  on  the  water,  see 
Isherwood's  calculation  on  the  "  San  Jacinto,"  (Journal  of  the  Franklin  Insti- 
tute, Third  Series,  Vol.  XXL,  p.  349,)  or  on  the  "  Arrogant,"  (Appleton's  Me- 
chanics' Magazine,  Vol.  I.,  p.  156,)  from  which  the  form  for  the  above  table  is 
taken. 


THE    SCREW    PROPELLER.  129 

instruments,  and  which  has  the  sum  of  its  losses  the 
least,  must  be  the  most  economical  propelling  instru- 
ment. The  feathering  wheel,  from  what  we  have 
seen,  would  present  itself  very  conspicuously  to  our 
eye  as  being  the  best  instrument  within  our  knowledge ; 
but,  unfortunately,  the  practical  difficulties  are  such  as 
to  preclude  its  universal  adoption.  The  loss  from  ob- 
lique action  in  the  common  radial  wheel,  particularly 
where  the  diameter  is  comparatively  small  and  the  dip 
of  the  paddles  considerable,  amounts  to  an  important 
percentage  of  the  total  power  of  the  engines;  and 
since  this  loss  in  the  screw  propeller  does  not  exist,  but 
is  replaced  by  one  of  much  smaller  magnitude,  viz., 
friction  of  the  blades,  it  follows,  that  were  the  slip  of 
the  two  instruments  alike,  the  screw  propeller  would 
be  the  more  economical.  In  practice,  however,  with 
the  screw  propeller,  when  contending  against  head 
winds,  or  other  increased  resistance,  the  slip  is  increased 
to  a  very  serious  extent.  In  fact,  in  some  cases  it  has 
occurred,  when  the  engines  were  going  ahead  at  nearly 
full  speed,  the  vessel  stood  nearly  still.  On  the  other 
hand,  however,  when  the  sails  are  set  to  a  fair  wind, 
the  slip  of  the  propeller  is  materially  reduced,  while 
the  thrust  remains  unaltered.  The  increased  slip 
when  contending  with  head  winds  is  also  experienced 
with  paddle  wheels,  but  they  are  not  affected  to  the 
same  extent  as  the  propeller,  the  increasing  or  decreas- 
ing the  resistance  with  the  latter  instrument,  not 
making  a  vast  difference  in  the  revolutions  of  the  en- 
gines (as  is  the  case  with  the  paddle  wheel)  so  long  as 
the  pressure  on  the  piston  remains  unaltered. 

In  the  application  of  the  sere  w  propeller,  it  is  well 
to  sink  it  as  low  as  possible  in  the  water,  in  order  that 
the  hydrostatic  pressure  above  may  be  sufficient  to 


130  THE    SCREW    PIIOPELLEE. 

cause  the  water  to  flow  in  solid,  even  to  the  centre  of 
the  propeller,  which,  therefore,  having  the  proper 
resisting  medium,  is  less  liable  to  excessive  slip.  This 
will  also  prevent  the  centrifugal  action — the  throwing 
of  the  water  off  radially  from  the  centre — which  exists 
to  a  small  extent  in  some  very  aggravated  cases. 

Increasing  the  helicoidal  surface  of  the  screw  be- 
yond what  is  barely  sufficient  to  transmit  the  power 
given  to  it,  has  no  other  effect  than  to  occasion  an 
increased  loss  by  friction,  by  the  increased  surface,  in- 
terposed. The  friction  of  solids  on  fluids,  unlike  solids 
on  solids,  depending  upon  the  extent  of  rubbing  sur- 
face as  one  of  the  elements.  The  object,  therefore,  to 
be  sought  after  in  practice,  is  to  make  the  sum  of  the 
loss  by  slip,  plus  friction,  as  little  as  possible,  and  this 
sum,  manifestly,  must  depend,  to  a  considerable  extent, 
on  the  amount  of  helicoidal  surface ;  but,  nevertheless, 
there  appears  to  be  no  general  rules  yet  devised,  from 
theory  or  practice,  which  can  be  used  as  a  reliable 
guide ;  different  engineers  making  considerable  differ- 
ence in  the  areas  of  propellers  applied  to  the  propulsion 
of  the  same  sized  and  modeled  steamers. 

Negative  Slip. — It  would  certainly  appear  a  very 
strange  anomaly,  were  one  on  board  a  vessel,  which  he 
should  discover  from  the  indications  of  the  log  was  mov- 
ing actually  faster  through  the  water  than  the  screw, 
there  being  no  other  propelling  instrument ;  yet  such 
has  been  apparently  the  case,  and  there  are,  perhaps,  to 
this  day,  persons — though  we  hope  they  are  very 
few — who  think  that  a  screw  propeller  may  drive  a 
vessel  faster  than  it  is  moving  itself.  There  have  been 
cases,  it  is  true,  where  the  log  has  shown  that  the  ves- 
sel was  apparently  moving  faster  than  the  screw,  which 


THE   SCEEW    PROPELLER.  131 

alone  was  the  propelling  instrument,  but  that  such  a 
thing  could  be  true  is  absolutely  absurd,  and  hence 
attention  was  turned  to  discovering  the  anomaly.  It 
is  accounted  for  in  two  ways. 

When  a  body  having  a  blunt  stern  is  drawn  through 
water  at  a  high  velocity,  the  water,  not  being  able  to 
flow  in  from  the  sides  of  the  body  sufficiently  rapid  to 
fill  the  vacuity  occasioned  by  its  passage,  flows  in  from 
all  other  directions,  and  a  column  of  water,  therefore, 
necessarily,  follows  in  the  wake  of  such  a  body.  This 
is  the  case  with  screw  propeller  vessels  having  blunt 
runs,  and,  by  consequence,  the  propeller,  instead  of 
acting  upon  water  at  rest,  acts  upon  water  in  motion, 
having  the  same  direction  as  the  vessel.  Now,  then, 
supposing  a  propeller,  acting  upon  water  at  rest,  to 
have  a  slip  of  10  per  cent.,  if  a  column  of  water  follow 
the  ship  with  the  velocity  of  11  per  cent,  of  the  speed 
of  the  propeller,  which  still  retains  its  ten  per  cent, 
slip,  the  log,  as  it  takes  no  cognizance  of  the  velocity 
of  this  water,  would  show  a  negative  slip  of  1  per  cent., 
i.  £.,  it  would  show  the  vessel  to  be  actually  moving  1 
per  cent,  faster  than  the  propeller,  when  in  reality  the 
latter  would  be  moving  10  per  cent,  the  faster. 

To  produce  such  a  result  as  this,  of  course,  possesses 
no  mechanical  or  other  advantage;  for  power  must 
have  been  originally  taken  from  the  engines  to  pro- 
duce the  current,  which  cannot  be  returned  to  its  full 
extent.  It  is,  therefore,  a  very  important  element  in 
the  design  of  a  screw  vessel  to  make  the  run  very 
sharp — the  lines  fine — in  order  that  the  water  may 
flow  in  solid  at  once,  to  fill  the  vacuity  occasioned  by 
the  vessel's  progress,  or  the  propeller's  revolutions. 

The  other  theory  in  regard  to  negative  slip  is  this : 
All  known  bodies  yield  to  pressure,  it  being  only 


132  ALTJKIMNOJ-    THE    PITCH. 

necessary  in  order  to  cause  the  amount  of  yield  to  be 
measurable  to  make  the  pressure  sufficiently  great. 
It  is  hence  conceived,  that  when  a  screw  propeller  is 
in  motion,  the  pressure  of  the  water  on  the  blades  causes 
them  to  spring,  thereby  increasing  the  pitch ;  conse- 
quently, in  calculating  its  speed  through  the  water,  if 
we  use  the  true  pitch,  instead  of  the  pitch  assumed, 
while  it  is  in  motion,  the  velocity  given  to  it  will  be 
too  small,  and  may  be  less  than  the  velocity  of  the 
vessel. 

We  would,  however,  remark,  that  negative  slip  in 
a  screw  propeller,  unassisted  by  sails,  is  more  imaginary 
than  real,  and  could  only  exist  under  very  aggravated 
circumstances,  for  a  screw  propeller  usually  has  about 
20  per  cent,  slip,  at  least,  and  to  reduce  this  to  nothing, 
even  under  the  conditions  set  forth  above,  would  be 
rather  a  perversion  of  circumstances. 

Altering  the  Pitch. 

Propellers  are  sometimes  constructed  in  such  a 
manner  that  the  pitch  can  be  altered,  from  time  to 
time,  by  altering  the  angle  of  the  blades,  which  are 
made  adjustable  in  a  large  spherical  hub.  Thus,  if  it 
be  desired  to  increase  the  pitch,  increase  the  angles  by 
turning  round  the  blades ;  or  if  it  be  desired  to  de- 
crease the  pitch,  reverse  the  operation.  Such  an  ar- 
rangement, however,  in  practice,  must  be  confined 
within  very  narrow  limits,  for,  inasmuch  as  the  surface 
of  a  screw  propeller  blade,  being  that  of  a  helicoid, 
every  point  in  the  blade  must  have  a  different  angle, 
which  increases  as  the  hub  is  approached,  and  if  the 
propeller  be  constructed  so  that  all  the  angles  be 
adapted  to  one  particular  pitch,  it  is  not  very  likely 


PARALLEL   MOTION.  133 

that  they  will,  after  being  distorted,  be  adapted  to  any 
other  pitch ;  that  is  to  say,  if  the  propeller  be  a  true 
screw,  for  instance,  and  have  a  certain  angle  at  the 
periphery,  if  we  move  the  blade  so  as  to  increase  the 
angle  at  that  point  10°,  the  angle  at  every  other  point 
in  the  blade  will  also  be  increased  10°,  which  should 
not  be  the  case,  but  should  be  correspondingly  less  as 
the  hub  is  approached;  thus,  by  this  arrangement,  we 
give  a  greater  pitch  at  the  hub  than  there  is  at  the 
periphery ;  and  if  the  operation  be  reversed,  and  we 
decrease  the  angle  at  the  periphery,  the  angle  at  the 
hub,  and  every  other  point  in  the  blade,  is  decreased 
to  precisely  the  same  extent,  thus  giving  less  pitch  at 
the  hub  than  there  is  at  the  periphery,  or  any  other 
point  in  the  blade.  "We  therefore  arrive  at  this  con- 
clusion : 

That  having  three  conditions  presented  to  us,  viz., 
true  screw,  expanding  screw,  from  periphery  to  hub, 
and  expanding  from  hub  to  periphery — the  latter  two 
not  in  regular  ratio — it  is  more  than  probable  that 
one  or  the  other  of  these  must  be  found  practically  to 
be  the  superior,  and  whichever  it  may  be,  and  that 
one  adopted,  the  advantage  to  be  derived  from  alter- 
ing it,  after  it  is  once  adopted,  does  not  appear  very 
plain,  the  arguments  to  the  contrary  notwithstanding. 


Parallel  Motion. 

Parallel  motion  is  a  combination  of  bars  and  rods, 
having  for  its  object  the  guiding  of  the  piston-rod  of  a 
steam-engine  in  a  constant  straight  line,  or  as  near  a 
straight  line  as  can  be  practically  attained.  It  is  ap- 
plicable, in  different  forms,  to  any  type  of  engine,  but 


134 


PARALLEL    MOTION. 


its  adaptation  to  the  side-lever  engine  is  the  more 
general. 

We  have  constructed  figure  10  with  the  view  of 


FIG.  10. 


illustrating  its  application  to  this  type  of  engine,  and 
to  clear  it,  if  possible,  of  the  mystery  that  usually 
hangs  over  it  in  the  shape  of  formulas.  A  B  is  half 
the  length  of  the  side  lever,  vibrating  on  the  centre  B ; 
A  C,  the  side  rod  attached  to  the  cross-head  at  C ; 
Gr  F,  the  parallel  motion  side  rod ;  D  F,  the  parallel 
bar,  and  E  F,  the  radius  bar  vibrating  on  the  centre  E. 
The  object  to  be  attained  is  to  make  the  point  C 
travel  vertically  in  a  straight  line,  or  as  near  so  as  pos- 
sible ;  and  from  the  construction  of  the  figure,  it  will 
be  seen  that,  when  the  point  Gr  moves  to  the  right  the 
point  F  moves  to  the  left,  and  vice  versa ;  hence  it  is 
manifest,  that  there  must  be  some  point  II,  in  the  rod 
F  Cr,  which  will  describe  very  nearly  a  straight  line, 
and  if  the  lengths  Gr  B  and  E  F  were  equal,  that  point 
would  be  in  the  centre  of  F  G  ;  but,  since  they  are  of 
unequal  lengths,  H  must  be  in  such  a  position  that 


PAEALLEL   MOTION.  135 

Now,  then,  having  secured  the  point  H,  draw  the 
line  B  C  through  H,  which  will  determine  C,  the  cen- 
tre of  the  cross-head ;  and  the  triangles  B  H  G,  B  C  A, 
being  similar,  and  joined  together  in  such  manner  that, 
no  matter  how  much  the  angles  of  the  one  may  alter, 
the  angles  of  the  other  must  alter 'to  precisely  the 
same  extent ;  and  hence,  these  triangles  always  remain- 
ing similar,  it  follows  that  if  the  apex  (H)  of  the  one 
moves  in  a  straight  line,  the  apex  (C)  of  the  other 
must  move  in  a  straight  line  also. 

It  matters  not  where  the  points  D  F  G  may  be 
situated,  so  long  as  D  does  not  coincide  with  C,  and 
the  figure  A  D  F  G  is  a  parallelogram ;  nor  does  it 
matter  about  the  respective  lengths  of  the  sides  of  the 
parallelogram,  so  long  asEFxFH  =  BGxGH. 

In  practice,  it  happens  sometimes  that  the  parallel 
motion  gets  out  of  adjustment,  the  piston  rod  perhaps 
rubbing  hard  on  one  side  of  the  stuffing-box  at  the  top 
of  the  stroke,  and  hard  on  the  opposite  side  on  the 
bottom  of  the  stroke ;  or  it  may  rub  hard  on  the  stuff- 
ing-box at  one  end  of  the  stroke,  and  be  quite  free  at 
the  other.  Such  a  result  can  be  brought  about  in 
three  ways  only :  either  the  sides  of  the  parallelogram 
A  D  F  G  have  got  out  of  parallelism,  the  radius  bar 
E  F,  of  incorrect  length  from  the  wear  of  the  brasses, 
<fec.,  or  the  centre  E  has  by  some  means  been  moved 
from  its  true  position. 

These  can  be  all  remedied  by  interposing  liners 
at  the  proper  places ;  of  course,  taking  care  about 
the  centre  A,  in  order  not  to  endanger  striking 
the  cylinder-head,  by  interposing  too  much  at  that 
point. 


136  STRENGTH    OF   MATERIALS. 


Strength  of  Materials. 

This  is  a  subject  which  does  not  properly  come 
within  the  province  of  the  present  Notes;  but  we 
have,  however,  thought  it  well  to  devote  a  short  space 
to  it  at  this  place,  confining  ourselves  to  a  few  practi- 
cal examples. 

Beams. — The  strength  of  beams  are  to  each  other 
directly  as  their  breadths  and  square  of  their  depths, 
and  inversely  as  their  lengths. 

EXAMPLE. — The  depth  of  the  beam  of  an  engine 
75  ins.  diameter  of  cylinder,  and  7  ft.  stroke,  at  centre 
is  42  ins.,  and  using  this  as  a  standard,  required  the 
depth  of  one  for  an  engine  of  80  ins.  diameter  of  cyl- 
inder, and  8  ft.  stroke,  the  breadth,  and  also  the  maxi- 
mum pressure  on  the  steam  piston  to  remain  the  same  ? 

ANSWER.— 7 52  x  7 :  802  x  8  :  :  422 :  2293.76  ins.  — 
square  of  the  depth ;  the  square  root  of  which,  47.9 
ins.,  is  the  depth  required. 

These  figures,  of  course,  do  not  apply  to  the  truss, 
but  to  the  solid  parabolic  beam. 

Shafts. — The  strength  of  shafts  to  resist  a  trans- 
verse, or  torsional  strain,  are  to  each  other  as  the 
squares  of  their  diameters ;  for  the  reason  that,  if  the 
diameter  of  a  shaft  be  doubled,  the  quantity  of  metal  is 
increased  fourfold,  which  would  occasion  the  strength 
to  increase  as  the  square,  but  at  the  same  time  there 
being  double  the  leverage  interposed  in  consequence 
of  the  double  diameter,  which,  being  multiplied  by 
the  square  (or  4),  will  give  the  cube  (or  8). 


STRENGTH    OF   MATERIALS.  137 

EXAMPLE. — The  shaft  of  a  steamer  is  17  inches 
diameter ;  cylinder,  75  inches  diameter ;  by  7  feet 
stroke  ;  required  the  diameter  of  a  shaft  for  a  steamer, 
having  an  engine  of  80  inches  diameter  of  cylinder,  by 
8  ft.  stroke,  taking  the  shaft  here  given  as  the  standard, 
the  maximum  pressure  on  both  steam  pistons  to  be 
alike. 

ANSWER.— 752  x  7  :  802  X  8  :  :  173 :  6388.459,  the 
cube  of  the  diameter,  the  cube  root  of  which,  18.55 
inches,  is  the  required  diameter  of  the  shaft. 

This  is  about  the  diameter  of  the  shafts  used  in 
practice  for  two  engines  of  80  inches  diameter  of  cyl- 
inder, and  8  feet  stroke  each.  The  proportion  in  prac- 
tice for  a  shaft  for  a  single  engine  of  this  size,  is  about 
15.5  ins.  diameter,  which  is  a  little  more  than  half  the 
strength  of  the  above  shaft,  owing  to  the  weight  of  the 
wheels,  <fec.,  (which  have  also  to  be  sustained  by  the 
shaft)  being  more  than  half. 

Screw  Propeller  Sliaft. — The  strain  on  the  shaft  of 
a  screw  propeller  is  of  two  kinds — one  in  line  with  the 
axis  tending  to  compress,  the  other  at  right  angles  to 
the  axis  tending  to  twist  it.  And,  inasmuch  as  the 
strength  of  a  shaft  to  resist  compression,  is  much 
greater  than  that  to  resist  torsion,  we  need  only  take 
the  latter  strain  into  consideration. 

Hence,  to  ascertain  the  diameter  of  a  screw  shaft, 
the  dimensions  of  the  propeller  and  thrust  being  given, 
let  A  B,  figure  11,  be  the  pitch,  B  C,  the  circumference 
at  centre  of  pressure,  and  A  C,  the  helix  for  one  con- 
volution at  centre  of  pressure.  Draw  B  D  at  right 
angles  to  A  C,  and  D  E  at  right  angles  to  B  C  ;  the 
lines  D  E  and  B  E  will  be  proportionally  the  com- 
pressional  and  torsional  strains  on  the  shaft ;  hence,  if 


138  STEEJSTGTH    OF   MATERIALS. 

B  E  be  multiplied  by  the  thrust  in  pounds,  and  divid- 
ed by  D  E,  the  quotient  will  be  the  pressure  in  Ibs., 
FIG.  11.  acting  at  the  centre  of  pressure 

of  the  blade  to  twist  the  shaft. 
This  pressure  being  multiplied  in- 
to the  leverage  of  the  centre 
of  pressure,  and  divided  by  the 
standard  of  the  metal  used,  will 
give  the  cube  of  the  shaft's  diame- 
ter, the  cube  root  of  which  will 
be  the  diameter.  But  since  the  triangles  A  C  B, 
B  D  E,  are  similar,  from  the  construction  of  the  figure, 
the  angles  being  respectively  equal,  the  sides  must  be 
proportional,  viz. :  A  C  to  B  D,  A  B  to  B  E,  and  B  C 
to  D  E.  Therefore,  having  the  lengths  of  the  two 
sides  A  B,  B  C,  of  the  triangle  ABC,  we  have 

VATB'xT+WCyTb       r 

—  =  diameter  ot  shaft  in  inches, 

o 

in  which  A  B  =  pitch  in  feet, 

B  C  =  circumference  at  centre  of  pressure  in 

feet, 

t  =  thrust  in  pounds, 
1)  —  distance  from  centre  of  shaft  to  centre 

of  pressure  in  feet, 

c  =  practical  coefficient  of  the  metal  used 
for  the  shaft,  per  sq.  inch  of  section 
for  a  leverage  of  one  foot. 

Paddle  Shafts. — EXAMPLE  1 . — Area  of  the  piston 
3848.4  sq.  ins. ;  maximum  pressure  per  sq.  inch  40  Ibs. ; 
stroke  10  feet;  one  engine;  required  the  diameter  of 
the  paddle  shaft,  the  practical  value  of  the  metal  being 
200  Ibs.  per  sq.  inch  of  cross-section,  with  a  leverage 
of  1  foot. 


STRENGTH    OF   MATERIALS.  139 

V  3848.4  X  40  X  5        ,_,_.        ,. 
ANSWER. -  =  15|  ins.  diameter. 

EXAMPLE  2. — Same  as  Example  1,  excepting  there 
are  two  engines  instead  of  one,  connected  at  right 
angles  ? 

ANSWER. — With  two  engines  connected  at  90°,  the 
position  in  which  the  greatest  pressure  on  the  shaft 
will  be  interposed  will  be  when  both  engines  are  in 
such  a  position  that  a  perpendicular,  let  fall  from  the 
centre  of  the  crank  upon  the  centre  line  passing 
through  the  centre  of  the  shaft,  will  enclose  an  angle 
of  45°,  which,  with  a  5  feet  crank,  will  give  a  leverage 
of  5  x  .TOm  (nat.  sin.  of  45°)  =  3.535  feet;  hence, 
supposing  the  pressure  at  this  position  of  the  engines 
to  be  40  Ibs.  per  square  inch,  we  have 


V  3848.4  X  40  X  3.535  ..-'.        ,. 

X  2  =  17.6  ins.  diameter. 


200 


Piston  Rods. — The  piston  rod  of  a  reciprocating 
steam-engine  is  subject  alternately  to  a  tensile  and 
compressing  strain ;  and  there  is  nothing  more  absurd 
than  the  rules  given  in  books  on  the  steam-engine,  de- 
fining its  diameter  to  be  a  certain  fraction  of  the  diam- 
eter of  the  cylinder,  independent  of  all  other  elements. 
For  instance,  suppose  a  rod  of  a  certain  diameter  and 
length  to  be  just  able  to  sustain  a  certain  weight 
placed  upon  the  top  of  it,  without  deflexion ;  it  is  ab- 
surd to  suppose  that  it  Would  sustain  the  same  weight 
if  the  rod  was  made  double  the  length,  retaining  the 
same  diameter ;  yet  the  rules  given  for  the  diameters 
of  piston  rods  are  regardless,  not  only  of  their  lengths, 
but  also  of  the  pressure  of  steam.  We  have,  therefore, 


140  STRENGTH    OF   MATERIALS. 

thought  it  well  to  copy  the  following  remarks  and  ta- 
ble from  Johnson's  translations  of  the  book  of  Indus- 
trial design,  by  M.  Armengaud,  the  elder,  and  M.  M. 
Armengaud,  the  younger: 

"  Compression  is  a  force  which  strives  to  crush,  or 
render  more  dense,  the  fibres  or  molecules,  of  any  sub- 
stance which  is  submitted  to  its  action. 

"  According  to  Rondelet's  experiments,  a  prism  of 
oak,  of  such  dimensions  that  its  length  or  height  is  not 
greater  than  seven  times  the  least  dimensions  of  its 
transverse  section,  will  be  crushed  by  a  weight  of  from 
385  to  462  kilogrammes  to  the  square  centimetre  of 
transverse  section,  or  a  weight  of  from  54*70  to  6547 
per  square  inch  of  transverse  section. 

"  In  general,  with  oak  or  cast  iron,  flexure  begins 
to  take  place  in  a  piece  submitted  to  a  crushing  force, 
as  soon  as  the  length  or  height  reaches  ten  times  the 
least  dimension  of  the  transverse  section.  Up  to  this 
point  the  resistance  to  compression  is  pretty  regular. 

"  Wrought  iron  begins  to  be  compressed  under  a 
weight  of  4900  kilog.  per  square  centimetre,  or  of 
nearly  70000  Ibs.  per  square  inch,  and  bends  pre- 
viously to  crushing,  as  soon  as  the  length  or  height  of 
the  piece  exceeds  three  times  the  least  dimension  of 
the  transverse  section.1' 

We  show,  in  the  following  table,  to  what  extent 
per  square  inch  we  may  safely  load  bodies  of  various 
substances : 


SURFACE   CONDENSERS. 


141 


Table  of  the  Weights  which  Solids — such  as  Columns,  Pilas- 
ters, Supports — will  Maintain  without  l)eing  Crushed. 


WOODS  AND  METALS. 


Description  of  Material. 

Proportion  of  Length  to  Least  Dimensions. 

Up  to  12. 

Above  12. 

Above  24. 

Above  48. 

Above  60. 

Sound  Oak  

Ibs. 
426.750 
270.275 
533.437 
137.982 
14225.000 
28450.000 
11707.175 

Ibs. 
355.625 
119.490 
440.975 
116.645 
11877.875 
23755.750 

Ibs. 
213.375 
71.125 
266.007 
69.709 
7112.500 
14225.000 

Ibs. 
71.125 

106.687 

2375.575 
4741.666 

Ibs. 
35.562 

1994.900 

2375.575 

Pitch  Pine  

Common  Pine  

Wrought  Iron  

Rolled  Conner...  . 

EXAMPLE.  —  What  is  the  least  diameter  of  a  piston 
rod  for  a  cylinder  having  a  cross-section  of  3848.4 
square  inches,  to  sustain  with  safety  a  pressure  per 
square  inch  of  piston  of  40  Ibs.,  the  proportion  of 
length  to  be  about  24  to  1  ? 

ANSWER.  —  Taking  one  half  the  number  in  the 
above  table  for  the  practical  value,  we  have 

'  =  43.28604  sq.  ins.  cross  section  of  the  rod, 

—  7.4  ins.  diameter  of  the  rod. 


-7.-,-, 

i  1  -L^.O  —7- 

43.28604 


Surface  Condensers. 

A  surface  condenser  is  an  instrument  for  condens- 
ing steam  by  contact  with  cold  metallic  surfaces,  in- 
stead of  bringing  it  directly  into  contact  with  a  shower 
of  cold  water.  The  object  of  using  such  a  condenser  in 
lieu  of  the  common  jet,  is  to  furnish  boilers  of  marine 
steamers  with  distilled  instead  of  sea  water,  conse- 


142  SURFACE    CONDENSERS. 

quently  to  provide  against  the  loss  of  fuel  otherwise 
occasioned  by  blowing  off  a  portion  of  the  water,  to 
keep  the  concentration  at  a  desired  point,  as  shown  at 
pages  66  and  67.  Also  to  prevent  the  loss  due  to  the 
little  conducting  power  of  the  envelope  of  scale  which 
attaches  to  all  heating  surfaces  of  boilers  using  sea 
water. 

By  the  use  of  such  an  instrument  there  is  also 
gained  the  saving  in  labor  of  scaling  and  cleaning  the 
boilers,  which  belongs  to  all  sea  steamers  using  the 
common  jet,  and  this  is  of  no  small  importance  to 
those  having  the  care  of  steam  machinery. 

Again,  by  its  use  the  expense  of  repairs  to  the 
boilers  is  considerably  reduced,  their  durability  great! y 
increased,  the  pressure  of  steam  which  can  be  ju- 
diciously carried  is  unlimited,  and  the  expansion  of  the 
steam  can  be  carried  to  a  greater  extent. 

With  these  many  marked  advantages,  it  seems  ex- 
traordinary that  the  introduction  of  surface  condensers 
should  have  met  with  so  little  encouragement ;  the 
slow  progress  made  has  not  been  owing  to  any  want 
of  engineering  ability,  but  solely  for  the  want  of  pat- 
ronage ;  for  engineers  of  talent  both  here  and  in 
Europe  have  devoted  their  time  to  the  subject  for 
many  years,  and  have  produced  many  forms,  some  of 
which  have  been  so  'successful  as  to  render,  in  our 
opinion,  the  use  of  jet  condensers  absurd.  Of  the 
number  invented  and  introduced  into  practice,  the  one 
known  as  Pirsson's  has  thus  far  met  with  the  most 
favor.  It  is  termed  a  double  vacuum  condenser,  i.  e., 
it  has  a  vacuum  within  and  without  the  condensing 
tubes.  The  injection  water  is  received  upon  a  scatter- 
ing plate,  and  showered  down  on  the  tubes,  which 
condenses  the  steam  within  them;  this  injection  water 


SURFACE    CONDENSERS.  143 

with  the  air  and  uncondensed  vapor  is  extracted  by 
au  air-pump,  in  the  same  manner  as  when  the  jet  con- 
denser is  used,  and  the  water  of  condensation  is  drawn 
away  by  a  separate  pump,  called  the  fresh  water  pump, 
and  discharged  into  a  reservoir,  whence  it  is  delivered 
by  the  feed-pumps  into  the  boilers. 

Another  variety  of  condenser,  known  as  Sewell's, 
has  recently  attracted  considerable  notice.  It  has  been 
highly  reported  upon  by  a  Board  of  naval  engineers 
appointed  by  the  Hon.  Secretary  of  the  Navy,  and  has 
been  introduced  into  some  of  the  most  successful 
steamers.  It  is  of  the  close  surface  type,  that  is,  it  has 
a  vacuum  upon  only  one  side  of  the  condensing  tubes, 
the  condensation  being  effected  by  currents  of  cold 
water  driven  through  the  tubes  by  a  pump.  The  joints 
of  the  tubes  are  made  with  india-rubber  sleeves,  so  that 
they  give  perfect  tightness,  and  allow  each  tube  to  ex- 
pand or  contract  by  itself,  independent  of  the  others, 
and  each  or  all  of  them  can  be  taken  out  for  cleaning 
or  repairs.  The  vacuum  produced  by  this  condenser 
is  unequalled,  and  as  there  is  but  one  air-pump,  it  is 
obtained  with  less  power  than  by  the  other  method. 

Close  tube  surface  condensers  had  been  made  with 
the  tubes  secured  at  both  ends  without  any  provision 
for  expansion  or  contraction,  except  the  buckling  of 
the  tubes  when  hot,  and  stretching  when  cold.  They 
have  also  been  made  with  one  end  of  the  tubes  only 
secured,  the  other  ends  being  fitted  to  an  expansion 
plate.  The  advantages  of  Mr.  Sewell's  over  the  latter 
named  plans  are  manifest. 

The  following  figure*  will  give  the  student  a  clearer 
idea  of  the  construction  and  operation  of  Pirsson's 
condenser.  A  A,  is  the  condenser,  in  which  there  is  a 

*  Taken  from  "Steam  for  the  Million'"  by  Commander  WARD,  U.  S.  N. 

10 


144 


SURFACE    CONDENSERS. 


series  of  small  tubes :  p,  the  air-pump ;  /,  fresh  water- 
pump  ;  ft,  the  exhaust  pipe ;  Z,  the  injection  pipe.    The 


operation  is  as  follows: — The  engine  being  put  in  mo- 
tion, the  exhaust  steam  flows  through  the  exhaust  pipe 
ft,  into  the  chambers  c  c,  thence  in  direction  of  the  ar- 
rows through  the  tubes  to  the  lower  chamber  d,  injec- 
tion water  being  admitted  at  the  same  time  from  the 
sea  through  the  injection  pipe  /,  is  showered  by  the 
scattering  plate  m  over  the  tubes,  and  by  its  gravity 
takes  the  direction  of  the  arrows  to  the  channel  way  ^, 
from  which  it  is  removed  by  the  air-pump  p,  and  de- 
livered into  the  hot  well  q  to  the  delivery  pipe  r  and 
overboard. 

The  water  resulting  from  condensation  is  drawn 
by  the  fresh  water  pumpy  from  the  chamber  J,  through 
the  pipe  e  e,  and  delivered  into  the  fresh  water  reser- 
voir g  j  from  this  reservoir  it  passes  to  the  feed  pump 
/,  through  the  pipe  ^,  and  is  delivered  into  the  boilers 
through  the  pipe  &.  The  pipe  s  is  for  the  purpose  of 
supplying  salt  water  when  deficiencies  occur. 

In  this  condenser,  as  drawn,  all  the  tubes  are  firmly 
secured  to  both  tube  heads,  but  one  end  of  the  tube 
box  is  free,  so  that  all  the  tubes  can  expand  and  con- 


CYLINDKICAL    BOILEES.  145 

tract  together ;  those  recently  constructed  have  each 
tube  secured  to  the  tube  head  at  one  head  only,  the 
other  ends  being  fitted  so  that  they  just  pass  through 
the  holes,  thus  allowing  each  tube  to  expand  or  con- 
tract regardless  of  the  others.  There  is  also  a  com- 
munication from  the  exterior  to  the  interior  side  of  the 
tubes,  so  that  the  vacuum  within  created  by  the  fresh 
water  pump  is  equal  to  that  without,  created  by  the 
large  air-pump.  In  close  tube  surface  condensers,  the 
position  of  the  steam  and  water,  as  shown  in  the  figure, 
is  reversed.  The  exhaust  steam  is  received  on  the  ex- 
terior of  the  tubes  as  at  £,  and  is  condensed  by  water 
entering  at  cf  c,  and  driven  through  thetubes  by  a  cir- 
culating pump,  attached  at  b  /  it  is  then  discharged 
through  a  pipe  from  d.  The  pump  p  is  converted  into 
a  fresh  water  air-pump,  receiving  the  fresh  water 
through  the  channel-way  n  and  foot-valve  o,  and  dis- 
charging it  into  the  reservoir  q,  whence  it  is  received 
by  the  feed-pump  and  pumped  into  the  boilers. 

Cylindrical  Boilers. 

The  force  tending  to  rupture  a  cylinder  along  the 
curved  sides  depends  upon  the  diameter  of  the  cylinder 

and  pressure  of  steam,  and  we 
may  regard,  hence,  the  total 
pressure  sustained  by  the  sides 
to  be  equal  to  the  diameter  x 
pressure  per  unit  of  surface  x 
length  of  boiler,  neglecting  any 
support  derivable  from  the  heads, 
which,  in  practice,  depends  on 
the  length.  The  shorter  the 
tube,  the  greater  its  powers  of  resistance.  This  is  in 


146  CYLINDRICAL    BOILERS. 

consequence  of  the  ends  being  rigid  and  unyielding. — 
See  latest  experiments  on  this  subject  by  William  Fair- 
bairn,  Esq.,  C.  E.,  F.  E.  S. 

The  force  tending  to  rupture  a  boiler  is  termed,  by 
Professor  Johnson,  the  divellant  force,  and  the  tenacity 
or  strength  of  the  metal  which  resists  the  divellant 
force  is  termed  the  quiescent  force.  When  rupture  is 
about  to  take  place,  these  two  forces  must  be  exactly 
equal. 

EXAMPLE. — What  pressure  will  a  cylinder  boiler, 
12  ins.  diameter,  and  i  in  thickness  of  metal,  sustain 
per  square  inch,  the  iron  to  be  of  the  best  English 
iron? 

The  experiments  of  the  Franklin  Institute  give  for 
the  strength  of  single  riveted  seams,  56  per  cent,  of 
the  sheet,  and  assuming  the  tensile  strength  of  the  best 
English  iron  to  be  60,000  Ibs.  per  square  inch  of  sec- 
tion, we  have 

60,000  x  .66 . — 7001b 

12  (diameter)  x  4  (length  of  band  to  make  1  sq.  in.  area  of  crosa  section)"' 

But  as  the  opposite  side  of  the  boiler  will  support  an 
equal  amount,  the  true  pressure  will  be  double  this,  or 
1400  Ibs.  per  square  inch,  one-fourth  of  which  only 
(350  Ibs.)  would  be  safe  to  subject  it  to  in  practice. 

From  this  we  see  that  the  bursting  pressure  of  a 
boiler  of  the  dimensions  above  given,  in  a  transverse 
direction,  is  1400  Ibs.  per  square  inch.  We  will  now 
see  what  force  this  1400  Ibs.  exerts  to  tear  the  boiler 
asunder  in  a  longitudinal  direction.  To  do  this,  we 
have  only  to  multiply  the  area  of  the  head  by  the 
pressure  per  square  inch,  and  divide  by  £  the  circum- 
ference, (since  the  iron  is  £  inch  thick,)  which  will 
give  the  strain  upon  each  square  inch  of  sectional  area. 


CYLINDRICAL   BOILERS.  147 

rp,        113.09  X  1400  .     , 

Inus  —  ,  - —  =  16800  IDS.  per  square  inch 

o • .oy  ~^~  -I 

of  sectional  area,  in  a  longitudinal  direction,  and 

1400  X  12  X  4  .     ,       „ 

-  =  33600  IDS.  per  square  inch  ot  sec- 
tional area  in  a  transverse  direction. 

The  4  in  the  latter  case  is  the  length  of  the  band 
to  give  one  inch  square  of  sectional  area,  and  we  divide 
by  2  because  there  are  two  sides  of  the  boiler  to  sup- 
port the  pressure. 

From  these  figures,  it  is  observed  that  the  strain 
upon  a  cylindrical  boiler,  or  other  cylindrical  vessels, 
subject  to  internal  pressure,  transversely,  is  exactly 
double  what  it  is  longitudinally.  In  cast  iron,  or  other 
cast  metal  cylindrical  vessels,  this  is  made  amends  for, 
in  a  certain  degree,  by  casting  ribs,  or  bands,  around 
the  external  surface ;  but  with  boilers  there  appears  to 
have  been  no  attempt  to  increase  the  strength  by  riv- 
eting bands  at  intervals  on  the  outer  surface,  though 
we  see  no  good  reason  why  such  a  thing  could  not  be 
done  very  advantageously. 

We  remark,  from  what  has  appeared,  that  the 
strain  upon  cylindrical  boilers  increases  transversely 
directly  as  the  diameters,  and  longitudinally  as  the 
squares  of  the  diameters — because  the  areas  of  the 
heads  increase  in  that  ratio — but  the  circumferences 
increase  also  as  the  diameters ;  and  hence,  though  we 
obtain  four  times  the  pressure  longitudinally  by  doub- 
ling the  diameter,  we  have  double  the  metal  in  the 
circumference  of  the  boiler  to  sustain  it,  and,  therefore, 
the  strain  upon  a  unit  of  metal,  in  this  direction,  in- 
creases also  as  the  diameter.  Hence,  no  matter  what 
may  be  the  diameter  of  a  boiler,  the  transverse  pres- 
sure tending  to  tear  it  asunder,  will  always  be  double 
the  longitudinal  pressure. 


148  BOILEK    EXPLOSIONS. 

Boiler  Explosions. 

There  is  only  one  grand  direct  cause  of  boiler  ex- 
plosions, and  that  is  the  incapacity  of  the  metal,  at  the 
time,  to  sustain  the  pressure  to  which  it  is  subjected. 
This  can  be  brought  about  in  several  ways  ;  defective 
material  of  which  the  boiler  is  constructed,  defective 
construction,  all  parts  of  the  boiler  being  incapable  of 
sustaining  the  same  pressure,  gradual  accumulated  pres- 
sure without  the  means  of  escape,  sudden  accumulated 
pressure  occasioned  by  pumping  water  on  red-hot  sheets, 
collapse  occasioned  by  a  vacuum  in  the  boiler,  the  re- 
verse valve  being  inoperative ;  collapse  of  flue  occa- 
sioned by  internal  pressure  in  the  boiler  and  a  partial 
vacuum  in  the  flue;  overheating  the  plates,  brought 
about  by  the  accumulation  of  large  quantities  of  scale 
upon  them,  thereby  reducing  their  tenacity. 

Boilers  having  been  previously  tested  by  hydros- 
tatic pressure  considerably  beyond  the  limit  to  which 
it  is  intended  ever  to  allow  the  steam  to  reach,  and 
each  and  every  boiler  being  fitted  with  steam  and  wa- 
ter-gauges, proper  sized  safety-valves  and  such  like  in- 
struments, there  is  never  any  good  excuse,  under  any 
circumstances,  for  the  cause  of  boiler  explosions.  In- 
competency  or  recklessness  must  be  somewhere  mani- 
fest, for  the  engineer,  knowing  the  pressure  which  his 
boiler  will  with  safety  bear,  should  under  no  circum- 
stances allow  it  to  exceed  that  pressure.  "We  would, 
however,  observe  here,  that  we  have  noticed  in  many 
cases,  both  ashore  and  afloat  where  there  are  a  number 
of  boilers  connected  together,  instead  of  having  a 
steam  gauge  attached  to  each  one  separately,  there  was 
but  one  gauge  to  the  whole  number ;  and  hence,  if  one 
or  more  boilers  be  shut  off  from  the  others,  there  would 


BOILER    EXPLOSIONS.  149 

be  no  means  of  ascertaining  the  pressure  within  them ; 
and  it  is  a  very  common  thing  with  land  boilers  and 
boilers  of  small  river  boats  to  have  no  steam-gauge 
whatever.  In  such  cases  as  these  the  owners  take 
upon  themselves  the  responsibility,  which  would  other- 
wise be  attached  to  the  engineer,  of  any  disastrous 
result. 

The  legislation  in  regard  to  the  inspection  of  steam- 
boilers  is  hardly  adequate  to  the  cause ;  for  though  the 
testing  the  strengths  of  boilers,  from  time  to  time,  is 
very  good  as  far  as  it  goes,  it  falls  short  of  what  the 
seriousness  of  the  case  demands.  The  same  amount  of 
strict,  unbiased  inspection  on  the  parties  who  have 
charge  of  the  very  powerful,  yet  governable  element 
of  steam,  would  be  followed  by  far  more  beneficial 
results.  Place  only  those  in  charge  of  the  steam- 
engine,  boilers,  and  dependencies,  who  are  competent 
to  the  task ;  prevent  owners  from  employing  any  one 
simply  because  his  services  can  be  secured  for  a  small 
compensation,  and  then  you  touch  the  subject  in  a  vital 
point.  It  is  too  prevalent  an  opinion,  that  any  one 
who  can  stop  and  start  an  engine,  have  the  fires  started 
and  hauled,  is  an  engineer,  regardless  of  his  knowledge 
of  the  element  of  which  he  has  charge. 

It  is  true,  however,  that  the  system  of  rivalry  and 
competition,  carried  on  by  steamboat  owners  and  others 
using  steam  power,  is  such  as  to  prevent  any  one  inde- 
pendently from  paying  a  very  high  rate  of  compensa- 
tion; but  if  all  were  compelled  to  employ  equally 
competent  services,  no  difficulty  could  be  experienced 
on  this  head. 


150  HOUSE   POWER. 

Horse  Power. 

The  standard  for  a  horse  power  in  England  and  the 
United  States  is  pretty  generally  established  at  33000 
Ibs.  raised  one  foot  high  in  a  minute ;  but  in  France  a 
horse  power  is  estimated  at  75  kilogramrnetres,  which  is 
75  kilogramme tres  raised  one  metre  high  per  second, 
equal  to  32554.7  Ibs.  avoirdupois,  raised  one  foot  high 
per  minute.  To  ascertain  the  horse  power  of  a  steam 
engine,  multiply  the  mean  unbalanced  pressure  per 
square  inch  on  the  piston,  by  the  area  of  the  piston  in 
square  inches,  by  the  length  of  the  stroke  in  feet,  and 
by  the  number  of  strokes  in  a  minute;  and  divide  by 
33,000,  the  quotient  will  be  the  horse  power. 

From  this  figure,  in  order  to  ascertain  the  actual 
power  utilized  in  propelling  the  vessel,  a  deduction  has 
to  be  made  for  working  the  air  and  feed  pumps  with 
their  load,  friction  of  working  journals,  friction  of  load 
on  working  journals,  amounting  in  all  to  about  20  per 
cent,  of  the  total  power,  leaving  80  per  cent,  to  be  ap- 
plied to  the  propelling  instrument,  which  80  per  cent, 
has  to  be  reduced  by  the  amount  of  loss  which  obtains 
in  the  propelling  instrument. 

EXAMPLE.^ — Required  the  horse  power  of  a  con- 
densing steam-engine,  having  a  cylinder  70  inches 
diameter,  by  10  feet  stroke,  making  15  revolutions  per 
minute  ;  mean  pressure  of  steam  throughout  the  stroke 
23  Ibs. ;  back  pressure  3  Ibs. ;  and  also  the  actual 
power  utilized  in  propelling  the  hull  of  the  vessel,  the 
sum  of  losses  in  the  propelling  instrument  being  40 
per  cent,  of  the  power  applied  to  it  ? 

ANSWER  IST.— 

702X.7854x23-3xlOxl5x2 

-33000-  --699.7  horse  power. 


HORSE    POWER.  151 

ANSWER  2o. — Considering  20  per  cent,  of  the  total 
power  to  be  expended  in  working  pumps,  in  friction, 
<fec.,  we  have  80  per  cent,  applied  to  the  propelling 
instrument,  and  40  per  cent,  of  80  per  cent.  =  32  per 
cent,  of  the  total  power  expended  in  transmission 
through  the  propelling  instrument ;  wherefore,  80  — 
32  =  48  per  cent,  of  the  total  power  applied  to  pro- 
pelling the  hull  of  the  vessel  =  335.856  horses. 

Nominal  Horse  Power,  is  a  term  which  expresses 
neither  the  actual  power,  the  size  of  the  engine,  nor 
any  thing  else  which  is  useful ;  and  though  it  has  be- 
come almost  obsolete  among  well-informed  engineers 
in  this  country,  our  trans-atlantic  friends  seem  yet -to 
cling  to  it  with  some  tenacity. 

The  usual  rule  for  determining  it  is  this :  Multiply 
the  square  of  the  diameter  of  the  cylinder  in  indies,  by 
the  cule  root  of  the  length  of  the  stroke  in  feet,  and 
divide  by  47  ;  the  quotient  is  the  horse  power. 

Now,  the  chief  object  for  establishing  a  rule  for 
nominal  horse  power  was  to  create  a  commercial  unit, 
by  which  the  power  of  one  engine  could  be  compared 
with  that  of  another  engine  ;  and  this  rule  might  meet 
the  wants  of  the  case,  did  the  lengths  and  breadths  of 
all  cylinders  bear  the  same  ratio,  and  did  the  pressure 
of  steam  remain  an  invariable  quantity :  but  as  these 
elements  are  constantly  varying,  it  is  of  no  use  what- 
ever ;  and  further,  if  they  did  not  vary,  the  simple 
square  of  the  diameter  would  express  an  unit  equally 
incorrect.  In  order  to  show  further  the  utter  useless- 
ness  of  the  term  horse  power,  as  expressed  above,  we 
will  take  two  engines,  each  having  YO  inches  diameter 
of  cylinder,  one  10  feet  stroke,  the  other  5  feet  stroke, 
and  ascertain  the  nominal  horse  power  of  each. 


152 


VIBRATION    OF    BEAMS. 


702X    V'lO 

47 

702X  V5 

47 


=  224.7  horses. 
=  178.2  horses. 


Now,  then,  if  the  pressure  of  steam  was  the  same 
in  these  two  cylinders,  and  the  pistons  moved  with 
the  same  velocity,  it  is  manifest  that  the  powers  must 
be  the  same ;  yet,  according  to  the  rule  for  nominal 
horse  power,  they  are  made  widely  different ;  and  if 
so  much  difference  is  made  while  the  pressure  of  steam 
is  supposed  to  remain  constant,  what  must  we  expect 
when  that  element  also  varies  ? 


Vibration  of  Seams. 

Given,  the  length,  O  c,  from  centre  of  beam,  to  a  &, 
line  passing  through  centre  of  cylinder  =  10  feet ;  and 


FIG.  ia 


length  of  stroke  =10  feet ;  required,  the  length  O  A, 
or  O  C,  of  half  the  beam  ? 


MARINE   ECONOMY.  153 

The  line  a  b  bisects  the  versed  sine  of  the  arc,  and, 
supposing  one  half  (c  C)  of  the  versed  sine  to  be  =  a?, 
we  have  (10  +  xf  =  (10  -  xf  +  52 

100  -J-200  +  a2  =100  —  20#  +  a?2  +  25 


#  =  .625, 

Hence,  half  the  length  of  the  beam  =  (10  +  .625) 
=  10.625  feet. 


Marine  Economy. 

A  body  moving  through  water  with  a  certain 
velocity  displaces  a  certain  quantity  of  water  in  a  given 
time,  with  a  certain  velocity  ;  if  the  velocity  be  doubled, 
the  quantity  of  water  displaced  will  also  be  doubled, 
because  the  body  moves  double  the  distance,  and  each 
particle  of  water  will,  therefore,  be  displaced  with 
double  the  velocity  ;  hence,  the  resistance  to  the  body 
will  be  as  2  X  2,  or  as  the  square  of  the  velocity.  Thus 
it  appears  that,  if  a  ship  consumes  500  tons  of  coal  to 
perform  a  certain  distance,  at  the  rate  of  5  miles  the 
hour,  to  perform  the  same  distance  at  the  rate  of  10 
miles  the  hour,  would  require  52  :  102  :  :  500  :  2000 
tons,  or  4  times  500  ;  but  the  quantity  of  coal  required 
for  any  one  day,  at  the  rate  of  10  miles,  will  not  be  4 
times  the  quantity  required  at  that  rate  for  5  miles, 
but  will  be  8  times  ;  for,  supposing  the  speed  be  in- 
creased to  10  miles  the  hour,  the  same  distance  will  be 
performed  in  5  days  ;  hence,  we  have,  in  the  first  case, 
500  tons  consumed  in  10  days  =  50  tons  per  day,  and 
in  the  latter  case,  2000  tons  in  5  days  =  400  tons  per 
day,  or  8  times  50  tons.  Now,  then,  taking  the  coal 
as  the  exponent  of  the  power,  we  see  that  the  power 


154  MAEINE   ECONOMY. 

has  to  increase  as  2x2x2,  or  as  the  cube  of  the 
velocity.  Hence  the  importance,  wherever  speed  is 
not  an  object,  of  running  the  engines  as  slow  as  possi- 
ble, in  order  to  economize  the  fuel. 

But  whenever  there  is  an  adverse  current  to  con- 
tend with,  the  most  economical  speed  is  half  as  fast 
again  as  the  current.  That  is  to  say,  if  the  velocity 
of  the  current  be  4  miles  the  hour,  the  velocity  of  the 
vessel  should  be  6  miles.  We  will  endeavor  to  demon- 
strate this  without  the  use  of  mathematical  formula. 

Let  1  represent  the  power  required  for  a  speed  of 
one  mile  per  hour,  then,  inasmuch  as  the  power  in- 
creases as  the  cube  of  the  velocity,  the  power  required 
for  the  speed  of  6  miles  =  63  =  216,  and  the  ground 
moved  over  =  4  —  2  —  2. 

Suppose,  now,  the  velocity  of  the  ship  be  reduced 
to  5  miles  per  hour,  the  power  will  be  =  53  =  125,  and 
the  ground  moved  over  =5  —  4  =  1. 

Suppose,  again,  the  speed  to  be  increased  to  7  miles 
per  hour,  the  power  will  be  =73  =  343,  and  the  ground 
moved  over  =  7  —  4  =  3. 

Summing  up  these  figures,  we  have  for  a  speed  of 
7  miles  per  hour  a  power  expended  of  343,  to  make 
good  a  distance  of  3  miles  =  114^  per  mile;  for  a 
speed  of  5  miles,  a  power  of  125  to  make  good  1  mile 
=  125  per  mile;  and  for  a  speed  of  6  miles,  216,  to 
make  good  2  miles  =  108  per  mile.  Consequently, 
the  least  power  is  required  at  the  speed  of  6  miles, 
which  is  half  as  fast  again  as  the  current. 

Had  the  calculation  been  made  for  any  fraction  of 
a  mile,  either  above  or  below  6,  the  same  result  would 
have  been  obtained. 

These  calculations  apply  alike  to  head  winds,  <fec., 
at  sea,  as  well  as  to  a  tide-way  in  a  river ;  whence  it 


LIMIT   TO    EXPANSION.  155 

follows  that  a  vessel  can  be  run  even  too  slow  for 
economy,  but  nevertheless,  when  having  a  heavy  head 
sea  to  contend  with,  there  are  other  elements  besides 
economy  of  fuel  to  be  taken  into  consideration ;  the 
strain  upon  the  vessel  and  machinery,  the  plunging 
and  staving  in  of  the  light  work  about  the  bows  and 
other  places,  shipping  of  seas,  <fec.,  are  matters  which 
also  require  the  judgment  of  the  commanding  officer. 

Limit  to  Expansion. 

Theoretically,  supposing  a  perfect  vacuum  to  ob- 
tain in  the  cylinder,  there  is  no  limit  to  expansion ; 
but,  practically,  there  is.  The  unbalanced  pressure  at 
the  end  of  the  stroke  should  never  be  less  than  suffi- 
cient to  overcome  the  friction  of  the  engine,  and  ought 
always  to  be  a  little  more. 

EXAMPLE. — Length  of  stroke  =  8  ft. ;  initial  pres- 
sure of  steam  30  Ibs.  per  square  inch,  inclusive  of  the 
atmosphere ;  back  pressure  4  Ibs.  per  sq.  inch  ;  friction 
of  engines,  <fec.,  =  2  Ibs.  per  square  inch ;  required  the 
point  where  the  steam  should  be  cut  off  to  yield  all  its 
useful  effect  ? 

x  =  the  point, 
4  +  2  =  6  =  the  pressure  at  the  end, 

x  X  30  =  6  X  8 
30a?=48 

x  =  1.6  ft.  from  commencement. 

Tlie  Proper  Lift  for  a  Valve 

Is  equal  to  the  area  of  the  valve  divided  by  the 
circumference. 


156 


GEAVITY. 


Centre  of  Gravity. 

The  centre  of  gravity  of  a  cone  from  the  vertex 
equals  f-  the  axis. 

In  a  paraboloid,  the  distance  from  vertex  equals  -f 
the  axis. 

In  a  parabolic  space,  equals  •§-  the  axis  from  the 
vertex. 

In  a  triangle,  equals  J-  the  axis  from  the  vertex. 


Centre  of  Pressure. 

The  centre  of  pressure  of  a  parallelogram,  when 
the  upper  surface  is  level  with  the  water,  =  \  from  the 
bottom  ;  of  a  right-angled  triangle  with 
the  base  down  —  \  from  the  bottom, 
measured  on  the  perpendicular  line 
B  C ;  with  the  base  up  —  ^  B  C.— See 
Hanris  Mechanics. 


Semi-parabolic  plane. 

FOKMULA : 

m  =  centre  of  pressure, 
b  m  =   \  of  a  (?, 
m  n  —  -/¥  of  a  d. 

Gravity. 


The  spaces  described  by  a  body  acted  upon  freely 
by  gravity  are  as  the  squares  of  the  times  ;  i.  e.,  a  body 
falling  2  seconds,  will  describe  4  times  the  distance  of 


GRAVITY.  157 

a  body  falling  one  second.  Hence,  in  order  to  ascer- 
tain the  distance  fallen  by  a  body,  it  is  only  necessary 
to  multiply  the  square  of  the  number  of  seconds  by 
the  distance  fallen  in  the  first  second  ;  the  product  will 
be  the  total  distance  fallen. 

All  bodies  fall  with  the  same  velocity  in  vacuo, 
namely,  16.08  feet  in  the  first  second,  having  a  velocity 
of  32.166  feet  at  the  end  of  the  second.  Where  the 
atmosphere  is  interposed,  the  velocity  will  be  some- 
what less,  say  for  heavy  bodies,  such  as  the  metals,  16 
feet  for  the  first  second. 

EXAMPLE. — Which  will  strike  with  the  greater 
'effect,  a  weight  of  200  Ibs.,  falling  through  144  ft.,  or 
100  Ibs.  falling  through  256  feet  ? 

The  velocity  of  a  body  at  the  end  of  a  fall  is  equal 
to  the  number  of  seconds  it  is  falling,  multiplied  into 
(32  feet)  the  velocity  at  the  end  of  the  first  second, 
and  the  momentum  of  a  body  is  equal  to  the  weight 
multiplied  into  the  velocity.  We  have,  then,  first  to 
find  the  velocity,  and  afterwards  the  momentum. 

v/16  :  1  :  :  v/144  :  3  seconds  time  of  falling  of  200  Ib. 
v/16  :  1  :  :  v/256  :  4  "  "  "  100  Ib. 

V, 

32  X  3     =       96  ft.  per  second  velocity  at  end  of  fall 

of200lb.  weight. 
32  X  4     =     128  ft.  per  second  velocity  at  end  of  fall 

of  100  Ib.  weight. 

96  X  200=19200=  momentum  of  the  200  Ib.  weight. 
128x100=12800=  momentum  of  the  100  Ib.  weight. 
6400=  difference,  which  is  33^-  per  cent, 
of  the  larger  number. 


158  DISPLACEMEISTT    OF    FLUIDS. 


Centre  of  Gravity  of  Several  Bodies  taken  together. 

Suppose  there  be  several  weights  placed  as  follows 
in  the  same  plane,  required  the  centre  of  gravity  of 
them  all  taken  together  ? 

Cylinder.  Air-pump.  Shaft.  Boilers. 

Tons.  Tons.  Tons.  Tons. 

5  2  10  30 

<       8ft.        x      10ft     x     20ft.    >         a 

Assume  a  point  (a),  at  any  distance  (say  2  feet) 
from  either  of  the  extreme  weights,  and  multiply  each 
weight  separately  by  its  distance  from  this  point ;  the 
sum  of  these  products,  divided  by  the  sum  of  the 
weights,  will  be  the  distance  of  the  centre  of  gravity 
from  the  assumed  point.  Thus : 

30  X  2    =    60 
10  X  22  =  220 

i>  x  32  =    64 

5  x  40  =  200 


47  )  544  (11.57  ft.  =  centre  of 

gravity  from  the  point  #,  or  9.57  feet  from  the  boilers 
towards  the  shaft. 


Displacement  of  Fluids. 

Solid  bodies  immersed  in  fluids  will  displace  an 
amount  of  the  fluid  equal  to  their  own  weight.  If  the 
specific  gravity  of  the  body  be  greater  than  that  of 
the  fluid,  it  will  sink  ;  otherwise  it  will  float. 

EXAMPLE. — Required  the  distance  a  cube  of  cherry, 
one  foot  high,  will  sink  in  fresh  water  ? 


TEMPERATURE  OF  CONDENSER.          159 

The  specific  gravities  of  fresh  water  and  cherry  are 
relatively  as  1.00  to  .606 ;  the  cherry  will  therefore 
sink  .606  feet. 

Temperature  of  Condenser. 

EXAMPLE. — Water  in  the  boilers,  carried  at  a  den- 
sity of  If  per  saline  hydrometer;  temperature  of  the 
condenser,  and  water  entering  the  boilers,  105°  Fahr. ; 
vacuum  in  condenser,  27.82  inches.  Compare  the  eco- 
nomic performance  of  the  engine,  under  these  circum- 
stances, with  the  same  engine,  carrying  the  water  in 
the  boilers  at  the  same  density,  but  the  water  in  the 
condenser  at  120°  Fahr. ;  the  mean  pressure  of  steam 
in  both  cases  on  the  piston  being  20  pounds  per  square 
inch? 

SOLUTION. — Neglecting  the  difference  of  power  in 
the  two  cases  required  to  work  the  air-pump,  taking 
the  boiler  pressure  at  20  Ibs.,  and  2  inches  of  mercury 
to  be  equal  to  1  Ib.  pressure,  we  proceed  thus  : 

1184-105 X  .75+228.5-105  :  228.5-105  :  :  100  : 
13.23  per  cent,  loss  by  blowing  off,  in  the  first  case. 

1184-120X.75+228.5-120  :  228.5-120  :  :  100  : 
11.96  per  cent,  loss  by  blowing  off,  in  the  second  case. 

20x2  :  2.18  (back  pressure)  :  :  100  :  5.45  per  cent, 
of  the  effect  of  the  engine  lost  by  back  pressure,  in  the 
first  case. 

20  X  2  :  3.33  (back  pressure) :  :  100  :  8.325  per  cent, 
of  the  effect  of  the  engine  lost  by  back  pressure,  in  the 
second  case. 
11 


160         TEMPERATURE  OF  CONDENSER. 

Now,  then,  letting  the  fuel  represent  the  power, 
we  observe,  in  the  first  case,  that  only  (100—13.23=) 
86.77  per  cent,  reaches  the  engine,  of  which  5.45  per 
cent,  is  lost  in  back  pressure,  and  5.45  per  cent,  of 
86.77  per  cent.=4.73  per  cent,  of  the  total  effect  lost 
by  back  pressure,  leaving  (86.77—4.73=)  82.04  per 
cent,  to  be  applied  to  operating  the  engine. 

In  the  second  case,  (100—11.96=)  88.04  per  cent, 
of  the  power  reaches  the  engine,  of  which  8.325  per 
cent,  is  lost  in  back  pressure,  and  8.325  per  cent,  of 
88.04  per  cent. =7.33  per  cent,  of  the  total  effect  lost 
by  back  pressure;  leaving  (88.04—7.33=)  80.71  per 
cent,  to  be  applied  to  operating  the  engine. 

Therefore,  under  the  conditions  of  the  example, 
the  engine,  in  the  first  case,  performs  the  same  amount 
of  work  with  (82.04—80.71=)  1.33  per  cent,  less  fuel. 

This  calculation  can  be  made  accurate  by  taking  dia- 
grams from  the  cylinder  and  air-pump,  under  the  con- 
ditions of  the  example,  and  estimating  the  power  in 
each  case ;  then,  the  power  to  work  the  air-pump  is 
considered. 


APPENDIX. 


MATEEIALS. 


IF  Engineers  possessed  the  proper  knowledge  of 
the  materials  used  in  the  construction  of  machinery, 
and  gave  the  attention  and  care  to  the  subject  which 
it  deserves,  we  should  have  fewer  break-downs  in  our 
sea-going  steamers  ;  and  might,  with  safety  and  great 
advantage,  reduce  the  weights  of  those  parts  made  of 
wrought  and  cast  iron. 

It  can  scarcely  be  expected,  however,  that,  with 
the  onerous  duties  of  constructing  many  kinds  of  ma- 
chinery, they  can  be  well  versed  in  the  manufac- 
ture of  every  variety  of  iron ;  but  every  engineer  hav- 
ing the  superintendence  of  construction  or  repairs, 
should  make  himself  familiar  with  the  materials  used, 
so  as  to  be  able  to  distinguish  good  from  bad ;  to  know 
the  difference  between  superior  and  inferior  brands  of 
pig  iron,  and  the  reasons  thereof.  The  difference  be- 
tween superior  charcoal  boiler  plate  and  that  made 
directly  from  the  bloomery  or  puddling  furnace.  Also, 
the  peculiarities  of  open  sand  moulding,  green  sand 
castings,  dry  sand  castings,  and  loam  moulding.  The 
manner  of  providing  for  expansion  and  contraction  in 
castings  and  forgings,  as  well  as  the  necessity  of  avoid- 


162  HOW    TO    TEST    IKOX. 

ing  the  process  of  cold-hammered  forgings.  We  throw 
out  these  few  hints  simply  with  a  view  of  calling  the 
attention  of  engineers  to  this  important  branch  of  the 
profession,  in  which  practice  alone  can  make  them 
proficient. 

*  To  Test  tlie  Quality  of  Bar  Iron. — Cut  a  notch 
on  one  side  with  a  cold  chisel,  then  bend  the  bar  over 
the  edge  of  an  anvil  at  sharp  angles.  If  the  fracture 
exhibits  long  silky  fibres,  of  a  leaden  gray  color,  co- 
hering together,  and  twisting  or  pulling  apart  before 
breaking,  it  denotes  tough,  soft  iron,  easy  to  work  and 
hard  to  break.  In  general,  a  short,  blackish  fibre,  in- 
dicates iron  badly  refined.  A  very  fine  close  grain 
denotes  a  hard  steely  iron,  which  is  apt  to  be  cold 
short,  but  working  easily  when  heated,  and  making  a 
good  weld.  Numerous  cracks  on  the  edges  of  the  bar 
generally  indicate  a  hot  short  iron,  which  cracks  or 
breaks  when  punched  or  worked  at  a  red  heat,  and 
will  not  weld.  Blisters,  flaws,  and  cinder  holes  are 
caused  by  imperfect  welding  at  a  low  heat,  or  by  iron 
not  being  properly  worked,  and  do  not  always  indicate 
inferior  quality. 

To  Test  Iron  when  Hot.— Draw  a  piece  out,  bend 
and  twist  it,  split  it  and  turn  back  the  two  parts,  to 
see  if  the  split  extends  up ;  finally,  weld  it,  and  observe 
if  cracks  or  flaws  weld  easily.  Good  iron  is  frequently 
injured  by  being  unskilfully  worked :  defects  caused 
by  this  may  be  in  part  remedied.  If,  for  example,  it 
has  been  injured  by  cold  hammering,  moderate  anneal- 
ing heat  will  restore  it. 

*  Ordnance  Manual,  1858. 


CAST   IRON    AND   STEEL.  163 

Cast  Iron. — There  are  many  varieties  of  cast  iron, 
differing  from  each  other  according  to  the  kind  of  fuel, 
of  ore,  and  the  temperature  of  the  blast  from  which 
the  pigs  are  made — the  pig  iron  being  known  as  char- 
coal cold  blast,  charcoal  hot  blast,  anthracite  cold 
blast,  and  anthracite  hot  blast.  The  former  is  much 
the  superior,  and  the  latter  the  inferior  varieties.  Be- 
sides these  general  divisions,  the  manufacturers  distin- 
guish more  particularly  the  different  varieties  of  pig 
metal  by  numbers,  according  to  their  relative  hard- 
ness :  for  instance,  No.  1  is  the  softest  iron,  has  a  dark 
gray  appearance ;  No.  2  is  harder,  close  grained,  and 
stronger  than  No.  1 ;  it  has  a  gray  color  also,  and  con- 
siderable lustre.  No.  3  is  still  harder  than  Ijb.  2  ;  its 
color  is  gray,  but  inclining  to  white  ;  it  is  principally 
used  for  mixing  with  other  irons.  No.  4  is  a  bright 
iron,  also  used  to  mix  with  other  irons. 

When  a  piece  of  iron  is  broken,  and  the  fracture 
presents  grains  very  large,  or  very  small,  and  a  dull 
earthy  aspect,  loose  texture,  dissimilar  crystals  mixed 
together,  it  indicates  an  inferior  quality. 

All  cast  iron  expands  forcibly  at  the  moment  of  be- 
coming solid,  and  again  contracts  in  cooling.  The 
color  and  texture  of  the  castings  depend  greatly  on 
the  size  of  the  casting,  and  the  rapidity  of  cooling. 
Care  should  always  be  taken  to  cool  them  slowly. 

Steel. — To  test  steel,  break  a  few  bars,  taken  at 
random,  make  tools  of  them,  and  try  them  in  the 
severest  manner. 


164 


TENACITY    OF   MATERIALS. 


Tenacity  of  Materials. 


Cast  Steel ...134,000  Ibs. 

'Swedish 72,000  "1  Experiments  by  Frank- 
Salisbury,  Conn 66,000  I  lin  Institute,  on  bars 

Bar-iron  -  Bellefonte,  Pa 58,500   [  whose   cross  section 

English 56,000  J  was    about   one-fifth 

Pittsfield,  Mass 57,0001  of  a  square  inch. 

Tig  metal 15,000 

r      .           Good  common  castings 20,000  Experiments  of  Maj.  W. 

"     Sp^S  fro^n  bead, j-jjjj          Sftpa^nt 

Cast  Steel 128,000          on  pieces  whose  cross 

.  ,  (  30,000          section  was  nearly  1 

Bronze-gun  metal j  42|000          &^^  ^ 

Copper,  cast,  (Lake  Superior) 24,138  , 

Brass 18,000 

„  S  Wrought 34,000 

C°PPer     \  Cast..?. 19,000 

Tin,  cast 4,800 

Zinc 3,500 

Platinum 56,000 

Silver. 40,000 

Gold , 80,000 

Lead 1,800 

WOODS. 

Ash 15,800 

Mahogany 11,500 

Oak 11,600 

White  Pine 11,800 

Walnut 7,700 

In  general,  the  tenacity  of  metals  is  increased  by 
hammering  and  wiredrawing.  The  strength  of  Pitts- 
field  bar  iron,  given  in  the  above  table,  is  the  mean  of 
four  trials,  with  cylinders  1  inch  long  and  0.9  inch  di- 
ameter. They  were  extended  in  length,  before  frac- 
ture, to  1.4  in.,  and  they  were  reduced  in  diameter  to 
0.6  in.  in  the  middle. 

A  bar  of  wrought  iron  is  extended  about  one-hun- 
dredth part  of  its  length  for  every  ton  of  strain  on  a 
square  inch. 

Transverse  Strength. 

S  =  the  weight  in  pounds  required  to  break  a  beam 
1  in.  square  and  1  in.  long,  fixed  at  one  end  and  loaded 
at  the  other ;  I  the  breadth,  d  the  depth,  and  I  the 


RESISTANCE   TO   TORSION.  165 

length,  in  inches,  of  any  other  beam  of  the  same  mate- 
rial, and  W  the  weight  which  will  cause  it  to  break, 
neglecting  the  weight  of  the  beam  itself. 

1.  If  the  beam  is  supported  at  one  end,  and  loaded  at  the  other : 

M* 
W  =  S  -y- 

2.  If  the  beam  is  supported  at  one  end,  and  the  load  distributed  over  its 

whole  length : 

b<P 
W  =  2S  -y 

3.  If  the  beam  is  supported  at  both  ends,  and  loaded  in  the  middle : 

bd* 
W  =  4S  y- 

4.  If  the  beam  is  supported  at  both  ends,  and  loaded  uniformly  over  its 
whole  length : 

bd? 
W  =  8S  y~ 

5.  If  the  beam  is  supported  at  both  ends,  and  loaded  at  the  distance  m  from 
one  end  : 

Ibd* 

W  =  S       /,         r 
m  (l—m) 

Resistance  to  Torsion. 

S  =  the  weight  in  pounds  required  to  break,  by 
twisting,  a  solid  cylinder,  1  inch  diameter ;  the  weight 
acting  at  the  distance  of  1  inch  from  the  axis  of  the 
cylinder ;  d,  the  diameter  in  inches  of  any  other  cylin- 
der of  the  same  material ;  r,  the  distance  from  its  axis 
to  the  point  where  the  breaking  weight  W  is  applied ; 
then: 

d3 


Results  of  Repeated  Heating  Bar  Iron. 

In  a  series  of  experiments,  with  regard  to  the  im- 
provements and  deterioration  which  result  from  oft- 
repeated  heating  and  laminating  of  bar  iron,  made  by 
William  Clay,  Esq.,  of  the  Mersey  steel  and  iron  works, 
Liverpool,  he  says  that,  taking  a  quantity  of  ordinary 


166        RESULTS    OF   EEPEATED   HEATING   BAK   IEON. 


fibrous  puddled  iron,  and  reserving  samples  marked 
No.  1,  we  piled  a  portion  five  high,  heated  and  rolled 
the  remainder  into  bars  marked  No.  2,  again  reserving 
two  samples  from  the  centres  of  these  bars,  the  remain- 
der were  piled  as  before,  and  so  continued  until  a  por- 
tion of  the  iron  had  undergone  twelve  workings. 

"  The  following  table  shows  the  tensile  strain  which 
each  number  bore : 

No.  Pounds. 

1.  Puddled  bar 43,904 

2.  Re-heated 52,864 


3. 

4. 

5. 

6. 

7. 

8. 

9. 
10. 
11. 
12. 


59,585 
59,585 
57,344 
61,824 
59,585 
57,344 
57,344 
54,104 
51,968 
43,904 


"  It  will  thus  be  seen  that  the  quality  of  the  iron 
increased  up  to  No.  6,  (the  slight  difference  of  No.  5 
may,  perhaps,  be  attributed  to  the  sample  being  slightly 
defective) ;  and  that  from  No.  6  the  descent  was  in  a 
similar  ratio  to  the  previous  increase." 

TENSILE  STRENGTH  OF  IRON  AND  STEEL  BARS  PER  SQUARE  INCH. 


Description  of  Iron  and  Steel. 

Tensile  Strength. 

Authority. 

62  644 

56,532 

American  Board  of 

56,103 

Ordnance. 

American  Hammered  

53,913 

Krupp's  Cast  Steel,  average  of  3  samples... 
Cast  Steel,  highest  

111,707 
142,222 

Min.  of  War,  Berlin. 
Mallett. 

88,657 

do. 

U                     <( 

134,256 

150,000 

Shear  Steel  

124,400 

Blister  "     

133,152 

Mersey  Steel  and  Iron  Co.  Puddled  steel, 
highest  

173  817 

Dito  another  sample  

160  8:32 

Average  of  three  samples  tested  at  the  Liv- 
erpool Corporation  testing  machine  

1  1  -V^O 

STRENGTH    OF   JOINTS    OF   BOILER   PLATES.  167 

On  the  strength  of  the  joints  of  single  and  double  riveted 
boiler  plates,  by  William  Fairlairn,  Esq.,  F.R.  S. 

On  comparing  the  strength  of  plates  with  their 
riveted  joints,  it  will  be  necessary  to  examine  the  sec- 
tional areas  taken  in  a  line  through  the  rivet-holes  with 
the  section  of  the  plates  themselves.  It  is  perfectly 
obvious,  that  in  perforating  a  line  of  holes  along  the 
edge  of  a  plate,  we  must  reduce  its  strength :  it  is  also 
clear  that  the  plate  so  perforated  will  be  to  the  plate 
itself,  nearly  as  the  areas  of  their  respective  sections, 
with  a  small  deduction  for  the  irregularities  of  the 
pressure  of  the  rivets  upon  the  plate ;  or,  in  other 
words,  the  joint  will  be  reduced  in  strength  somewhat 
more  than  in  the  ratio  of  its  section  through  that  line 
to  the  solid  section  of  the  plate.  It  is  evident  that  the 
rivets  cannot  add  to  the  strength  of  the  plates,  their  ob- 
ject being  to  keep  the  two  surfaces  of  the  lap  in  contact. 

When  this  great  deterioration  of  strength  at  the 
joint  is  taken  into  account,  it  cannot  but  be  of  the 
greatest  importance  that  in  structures  subjected  to  such 
violent  strains  as  boilers  and  ships,  the  strongest 
method  of  riveting  should  be  adopted.  To  ascertain 
this,  a  long  series  of  experiments 
were  undertaken  by  Mr.  Fairbairn, 
some  of  the  results  of  which  will 
be  of  interest  here.  The  joint  or- 
dinarily employed  in  ship  building 
is  the  lap  joint,  shown  in  Figs.  1 
and  2.  The  plates  to  be  united 
are  made  to  overlap,  and  the  rivets 
are  passed  through  them,  no  cov- 
ering-plates being  required,  except 


a  «,  a  •  •  a  9 


at  the  ends  of  the  plate,  where  they  butt  against  each 


168  STRENGTH    OF   JOINTS    OF   BOILER   PLATES. 

other.  It  is  also  a  common  practice  to  countersink  the 
rivet-heads  on  the  exterior  of  the  vessel,  that  the  hull 
may  present  a  smooth  surface  for  her  passage  through 
the  water.  This  system  of  riveting  is  only  used  when 
smooth  surfaces  are  required ;  under  other  circum- 
stances, their  introduction  would  not  be  desirable,  as 
they  do  not  add  to  the  strength  of  the  joint,  but,  to  a 
certain  extent,  reduce  it.  There  are  two  kinds  of  lap- 
joints,  those  said  to  be  single-riveted  (Fig.  1),  and  those 
which  are  double-riveted  (Fig.  2).  At  first,  the  former 
were  almost  universally  employed,  but  the  greater 
strength  of  the  latter  has  since  led  to  their  general 
adoption  in  the  larger  descriptions  of  vessels.  The  rea- 
son of  the  superiority  is  evident.  A  riveted  joint  gives 
way  either  by  shearing  off  the  rivets  in  the  middle  of 
their  length,  or  by  tearing  through  one  of  the  plates 
in  the  line  of  the  rivets.  In  a  perfect  joint,  the  rivets 
should  be  on  the  point  of  shearing  just  as  the  plates 
were  about  to  tear ;  but  in  practice,  the  rivets  are 
usually  made  slightly  too  strong.  Hence,  it  is  an  estab- 
lished rule,  to  employ  a  certain  number  of  rivets  per 
lineal  foot.  If  these  are  placed  in  a  single  row,  the 
rivet-holes  so  nearly  approach  each  other,  that  the 
strength  of  the  plates  is  much  reduced ;  but  if  they 
are  arranged  in  two  lines,  a  greater  number  may  be 
used,  and  yet  more  space  left  between  the  holes,  and 
greater  strength  and  stiffness  imparted  to  the  plates  at 
the  joint. 

The  experiments  of  Mr.  Fairbairn  and  others  have 
established  the  following  relative  strengths  as  the 
value  of  plates  with  their  riveted  joints : 

Taking  the  strength  of  the  plate  at 100 

The  strength  of  the  double-riveted  joint  would  then  be 70 

And  the  strength  of  the  single-riveted  joint 50 


MOTION.  169 


THE   ELEMENTS   OF  MACHTKEEY. 

IN  consequence  of  having  found  many  young  en- 
gineers unacquainted  with  the  principles  of  mechani- 
cal powers,  we  have  thought  best  to  devote  a  short 
space  to  the  subject,  prefacing  it  with  the  description 
of  motion,  and  application  of  power,  by  David  A. 
Wells,  A.  M. 

Motion. 

Motion  is  the  act  of  changing  place.  It  is  absolute 
or  relative.  Absolute  motion  is  a  change  of  position 
in  space,  considered  without  reference  to  any  other 
body.  Relative  motion  is  motion  considered  in  rela- 
tion to  some  other  body,  which  is  either  in  motion  or 
at  rest. 

When  a  body  commences  to  move  from  a  state 
of  rest,  there  must  be  some  force  to  cause  its  motion, 
and  this  force  is  generally  termed  "  Power."  On  the 
contrary,  a  force  acting  to  retard  a  moving  body,  de- 
stroy its  motion,  or  drive  it  in  a  contrary  direction, 
is  termed  "  Resistance."  The  chief  causes  which  tend 
to  retard  or  destroy  the  motion  of  a  body  are  gravi- 
tation, friction,  and  resistance  of  the  air. 

The  speed,  or  rate,  at  which  a  body  moves,  is 
termed  velocity.  The  momentum  of  a  body  is  its 
quantity  of  motion,  and  this  expresses  the  force  with 
which  one  body  in  motion  would  strike  against 
another.  This  momentum,  or  force,  which  a  moving 
body  exerts,  is  estimated  by  multiplying  its  weight 
by  its  velocity.  Thus  a  body  weighing  20  Ibs.,  and 
moving  with  a  velocity  of  200  feet  per  second,  will 
have  a  momentum  of  20  x  200  =  4000. 


170  APPLICATION    OF   POWER. 

Action  and  Reaction. 

"When  a  body  communicates  motion  to  another 
body,  it  loses  as  much  of  its  own  momentum,  or  force, 
as  it  gives  to  the  other  body.  The  term  Action  is 
applied  to  designate  the  power  which  a  body  in  mo- 
tion has  to  impart  motion,  or  force,  to  another  body ; 
and  the  term  Reaction  to  express  the  power  which 
the  body  acted  upon  has  of  depriving  the  acting  body 
of  its  force  or  motion.  There  is  no  motion,  or  action 
without  a  corresponding  and  opposite  action  of  equal 
amount ;  or,  in  other  words,  action  and  reaction  are 
always  equal  and  opposed  to  each  other. 

Application  of  Power. 

The  principal  agents  from  whence  we  obtain 
power  for  practical  purposes,  are  men  and  animals, 
water,  wind,  steam,  and  gunpowder. 

When  work  is  performed  by  any  agent,  there  is 
always  a  certain  weight  moved  over  a  certain  space, 
or  resistance  overcome ;  the  amount  of  work  per- 
formed, therefore,  will  depend  on  the  weight,  or  re- 
sistance that  is  moved,  and  the  space  over  which  it  is 
moved.  For  comparing  different  quantities  of  work 
done  by  any  force,  it  is  necessary  to  have  some  stand- 
ard ;  and  this  standard  is  the  power,  or  labor,  ex- 
pended in  raising  a  pound  weight  one  foot  high,  in 
opposition  to  gravity. 

A  machine  is  an  instrument,  or  apparatus,  adapted 
to  receive,  distribute,  and  apply  motion  derived  from 
some  external  force  in  such  a  way  as  to  produce  a 
desired  result ;  but  it  cannot,  under  any  conditions, 
create  power,  or  increase  the  quantity  of  power  or 
force  applied  to  it.  Perpetual  motion,  or  the  con- 
struction of  machines  which  shall  produce  power 
sufficient  to  keep  themselves  in  motion  continually,  is, 


APPLICATION   OF   POWER.  171 

therefore,  an  impossibility,  since  no  combination  of  ma- 
chinery can  create,  or  increase,  the  quantity  of  power 
applied,  or  even  preserve  it  without  diminution. 

The  great  general  advantage  that  we  obtain  from 
machinery  is,  that  it  enables  us  to  exchange  time  and 
space  for  power.  Thus,  if  a  man  could  raise  to  a  cer- 
tain height  200  pounds  in  one  minute,  with  the 
utmost  exertion  of  his  strength,  no  arrangement  of 
machinery  could  enable  him  unaided  to  raise  2000 
pounds  in  the  same  time.  If  he  desired  to  elevate 
this  weight,  he  would  be  obliged  to  divide  it  into  ten 
equal  parts,  and  raise  each  part  separately,  consuming 
ten  times  the  time  required  for  lifting  200  pounds. 
The  application  of  machinery  would  enable  him  to 
raise  the  whole  mass  at  once,  but  would  not  decrease 
the  time  occupied  in  doing  it,  which  would  still  be 
ten  minutes. 

The  power  will  overcome  the  resistance  of  the 
weight,  and.  motion  will  take  place  in  a  machine, 
when  the  product  arising  from  the  power  multiplied 
by  the  space  through  which  it  moves  in  a  vertical 
direction,  is  greater  than  the  product  arising  from 
the  weight  multiplied  by  the  space  through  which  it 
moves  in  a  vertical  direction.  Thus  if  a  small  power 
acts  against  a  great  resistance,  the  motion  of  the  lat- 
ter will  be  just  as  much  slower  than  that  of  the 
power,  as  the  resistance  or  weight  is  greater  than  the 
power,  or  if  one  pound  be  required  to  overcome  the 
resistance  of  two  pounds,  the  one  pound  must  move 
over  two  feet  in  the  same  time  that  the  resistance, 
two  pounds,  requires  to  move  over  one. 

All  machines,  no  matter  how  complex  and  intri- 
cate their  construction,  may  be  reduced  to  one  or 
more  of  six  simple  machines,  or  elements,  which  we 
call  the 


172  THE   LEVEE. 

Mechanical  Powers. 

The  simple  machines,  six  in  number,  are  usually 
denominated  the  lever,  inclined  plane,  wheel  and 
axle,  pulley,  screw,  and  wedge. 

The  wheel  and  axle  is,  however,  a  revolving  lever, 
the  screw  a  revolving  inclined  plane,  and  the  wedge 
a  double  inclined  plane,  thus  reducing  them  to  three 
in  number,  viz. :  lever,  inclined  plane,  and  pulley. 

All  these  machines  act  on  the  same  fundamental 
principle  of  virtual  velocities ;  accordingly,  tlie  weight 
multiplied  into  the  space  it  moves  through  is  equal  to 
the  power  multiplied  into  the  space  it  moves  through. 
This  is  the  general  law  which  determines  the  equi- 
librium of  all  machines ;  and  keeping  this  principle 
in  mind,  there  will  be  no  difficulty  in  solving  any  of 
the  propositions  appertaining  to  the  simple  machines. 

In  all  machines,  a  portion  of  the  effect  is  lost  in 
overcoming  the  friction  of  the  working  parts;  but, 
in  making  calculations  upon  them,  it  is  made  first  as 
though  no  friction  existed,  a  deduction  being  after- 
wards made.  And  so  also  we  have  to  assume  a  per- 
fection in  the  machine  itself  which  does  not  exist ; 
that  is  to  say,  the  inclined  plane,  screw,  wedge,  <fec., 
to  be  a  perfectly  smooth  hard  inflexible  substance, 
and  the  rope  of  the  pulley,  and  wheel  and  axle,  to  be 
perfectly  flexible  and  non-elastic,  conditions,  for  which 
allowance  has  to  be  made  after  the  calculation  is 
completed. 

Lever. — Of  the  lever  there  are  three  orders,  as 
shown  respectively  by  the  figures  1,  2,  3. 


THE  LEVEK. 


m 


Tig.  2 

3 


0 


Tig.  3 


zy 


W  =  weight,  P  =  power,  F  =  fulcrum. 

EXAMPLE  1.  —  Given  the  Weight  W  =  1000  Ibs., 
required  the  power  P,  the  lengths  of  the  arms  re- 
spectively as  marked  in  the  figures  ? 

ANS.  1.—  P  x  3  =  1000  X  1 
3P  =  1000 
P  =  333    Ibs. 


•  Aire.  2.—  P  x  4  =  1000  X  1 
4P  =  1000 
P  =  250  Ibs. 

ANS.  3.—  P  X  1  =  1000  X  4 
P  =  4000  Ibs. 

EXAMPLE  2.  —  Given  a  compound  lever  with  lengths 


and  weight  as  marked  in  fig.  4,  required  the  power  P. 


174  THE    LEVEE. 

p  X  16  =  1000  X  4 
16p  =  4000 

p  =  250  Ibs.  =  weight  required  at 
p,  supposing  there  to  be  but  one  lever — therefore 

P  X  10  =  250  X  2 
10P  =  500 

P  =  50  Ibs. 
Or, 

1000  x4x2i=Px!Ox!6 
8000  =  160P 
P=  50 

EXAMPLE  3. — Given,  as  per  figure  5,  a  safety  valve 

Tig.  5. 
<   s     x  ao 


100  sqr.  ins.  area 

20  Ibs.  per  sq.  in.  pressure 
2000  Ibs.  total  pressure. 

100  sq.  ins.  area,  subject  to  a  pressure  per  square  inch 
above  the  atmosphere  of  20  Ibs.,  lengths  of  the  long 
and  short  arms  of  the  lever  as  shown  in  the  figure, 
required  the  weight  W  to  balance  the  pressure  on  the 
valve  ? 

W  x  25  =  100  X  20  X  5 
25W  =  10000 
W  =  400  Ibs. 

EXAMPLE  4. — Suppose,  in  example  3,  the  valve  and 
stem  should  weigh  20  Ibs.,  and  the  lever,  which  is 
uniform  throughout  its  length,  weigh  2  5  Ibs.,  what 
would  be  the  weight  W,  in  that  case,  to  balance  the 
same  pressure  of  steam  ? 

The  valve  and  stem  being  5  inches  from  the  ful- 
crum, act  with  a  leverage  of  5  inches,  but  the  lever 
being  uniform,  its  action  is  the  same  as  though  the 


INCLINED    PLANE. 


whole  weight  was  concentrated  at  x  (the  centre  of 
gravity)  half  way  of  its  length,     Wherefore 

W  X  25  +  20  X  5  +  25  X  12.5  =  100  X  20  x  5 
25W  +  100  +  312.5  =  10000 

25 W  =  10000 -412.5 

W  =  383.5  Ibs.  the  re- 
quired weight. 

Practically,  the  pressure  a  safety  valve  lever  ex- 
erts on  the  valve  can  be  ascertained  by  fixing  it  in  its 
place,  and  attaching  a  spring  balance  to  the  pin  hole 
immediately  over  the  valve.  If  the  valve  and  weight 
be  also  attached,  the  balance  will  indicate  the  total 
pressure  which  tends  to  keep  the  valve  in  its  seat, 
which  pressure  being  divided  by  the  number  of  square 
inches  in  the  valve,  will  give  the  pressure  per  square 
inch  at  which  steam  will  commence  to  blow  off. 

Inclined  Plane. 
Ex.  1.— Weight  W  500  Ibs., 
length,  and  height  of  the  plane, 
as  per  figure  6,  20  and  9  ins. 
respectively,  required  the  pow- 
er P? 

Considering  the  weight  W  to  be  started  at  the 
base  of  the  plane  and  rolled  up  to  the  top,  it  will 
travel  vertically  the  height  of  the  plane,  (9  inches), 
while  the  power,  P,  will  descend  a  distance  equal  to 
the  length  of  the  plane  (20  ins.),  therefore,  according 
to  the  principle  of  virtual  velocities, 

P  x  20  =  500  X  9 
20P  -  4500 
P  =  225  Ibs. 

QJP  Ex.  2. — Length  and 
height  of  the  plane  as 
per  fig.  7,  weight  500 
pounds,  rpnuired  the 


176 


INCLINED    PLANE. 


power  P  applied  in  a  line  with  the  base  of  the 
plane  ? 

In  this  case,  when  the  weight  will  have  risen  from 
the  base  to  the  top  of  the  plane,  9  ins.,  the  distance 
descended  by  P  will  manifestly  not  be  equal  to  the 
length  but  to  the  base.  Wherefore 

P  X   v/20a-9'=  500  X  9 
17.86P  =  4500 

P  =  251.96  -  Ibs. 

In  order  to  establish  Equilibrium  between  the 
weight  and  power,  this  calculation  is  also  applicable 

A.  when  the  power  is 
applied  in  the  di- 
rection of  the  base 
as  shown  in  dots, 
figure  7. 

O  If  the  power  be 
applied  at  an  angle 
with  the  plane,  as 
C  A,  figure  8,  in 
order  to  ascertain  the  proportion  of  weight  to  the 
power,  to  establish  equilibrium,  we  proceed  thus : 
Draw  CD,  the  vertical  of  the  centre  of  gravity  of  the 
weight,  of  any  convenient  length ;  CE,  at  right  angles 
to  BF,  and  DE  parallel  to  AC.  CD  can  represent 
the  length  of  the  plane,  and  DE  the  height.  Where- 
fore 

Weight  x  DE  =  Power  x  CD 
Power  =  Weight  X  DE 

~CD~ 

Geometrically,  the  angles  B«C  and  CDE,  from 
the  construction  of  the  figure,  can  be  demonstrated 
to  be  equal,  and  also  ECD,  and  BFG ;  from  which, 
knowing  the  lengths  of  two  legs  of  the  triangle  BFG, 


WHEEL    AND    AXLE. 


177 


and  the  angle  G,  to  be  a  right  angle,  the  lengths  of 
the  lines  CD  ED  can  be  determined. 

Wheel  and  Axle. — In  the  wheel  and  axle,  when 
the  power  is  applied  tangentially  to  the  wheel, 

W  x  radius  of  axle  =  P  x  radius  of  wheel 
W  X  diameter  of  axle  =  P  x  diameter  of  wheel 
W  X  circumference  of  axle  =  P  x  circum.  of  wheel. 


When  the  power  is  not  applied 
tangentially  to  the  wheel,  but  in  the 
direction  shown  in  fig.  9,  the  length 
of  the  line  ah  at  right  angles  to  the 
power  will  give  the  leverage  of  the 
power, — hence 

W  x  radius  of  axle  =  P  x  oh. 


Tig.  9. 


Pulley. — If  a  cord  be  pulled  at  one  end  the  ten- 
sion throughout  its  whole  length  must  be  alike. 
Taking  figure  10,  and  supposing  the  power  to  be  1, 
the  tension  throughout  the  entire 
length  of  the  cord  will  be  1,  but 
as  there  are  two  parts  of  the  cord 
supporting  the  lower  block,  the 
weight  must  be  2.  The  pressure 
on  the  fulcrum  or  support  must 
be  always  equal  to  the  weight, 
plus  the  power.  If  there  be 
more  than  one  support,  the  sum 
of  the  pressures  on  them  will  be 
equal  to  the  sum  of  the  weight 
and  power.  Or,  in  figure  10, 
according  to  the  principle  of  virtual  velocities,  the 
weight  is  double  the  power,  because  the  power  must 
descend  2  feet  for  every  foot  ascent  of  the  weight. 


.  10 


178 


THE    PULLEY. 


The  numbers  above  the  top  blocks  in  all  the  ex- 
amples of  pulleys  here  shown  represent  the  pressure 
on  the  supports. 

In  fig.  11,  the  power  and  weight  are  as  1  to  8,  because 
the  power  supports  4  weights,  each  one  double  its  size. 


In  fig.  12  the  tension 
on  the  1st  cord  is  1 ;  on 
the  2d  2  ;  3d  4 ;  4th  8  ; 
5th  16 ;  and  as  there  are 
2  parts  of  the  cord  hav. 
ing  a  tension  of  16,  the 
weight  to  establish  equi- 
librium, must  be  32. 

In  fig.  13  the  weight 
to  the  power  is  as  3  to 
1,  there  being  3  parts 
of  the  cord  having  a 
tension  of  1  supporting 
the  weight. 


THE   PULLEY. 


179 


In  fig.  14  the  power 
to  the  weight  is  as  1 
to  12,  the  power  being 
multiplied  four  times 
by  the  application  of 
the  second  set  of  pul- 
leys, or  luff-tackles, 
as  they  are  technically 
termed. 

In  fig.  15  the  power 
is  to  the  weight  as 
1  to  12,  the  tension 
throughout  the  first 
cord  being  1 ;  the  sec- 
ond cord  2 ;  third  5, 
and  as  there  are  two 
parts  of  the  cord  hav- 
ing a  tension  of  5,  and 
one  part  of  the  cord 
having  a  tension  of  2, 
supporting  the  weight, 
if  all  the  cords  be 
supposed  parallel,  the 
weight  must  be  the 
sum  of  these,  or  12. 

In  fig.  16  the  power 
to  the  weight  is  as  1 
to  4. 

In  figure  17,  where 
the  power  is  applied 
at  an  angle,  we  ascer- 
tain the  proportion  of 
the  weight  and  power 
thus:  Draw  AD,  of 
any  convenient  length^ 
and  from  the  point  A 
draw  AB  parallel  to 


180 


THE  PULLEY. 


Cc  and  AC  parallel  to  BZ>.  The 
power  and  weight  will  be  re- 
spectively as  the  lengths  of  the 
lines  DC  or  DB  and  AD. 


rig.  16 


Eig-,17. 


From  which  it  will  be  seen  that  the  greater  the 
angle  CDB  the  longer  will  be  the  line  DC  or  DB,  and 
hence  the  greater  the  power.  So  that  the  weight  of  the 
line  itself  will  be  sufficient  to  prevent  any  power 
whatever  from  drawing  it  mathematically  straight. 


QUESTION. — In  figure  18,  two  blocks  of  granite, 
joined  together  as  shown,  are  laid  upon  a  horizontal 
plane ;  required  their  relative  sizes  in  order  that  they 
may  commence  at  the  same  time  to  move,  and  con- 
tinue to  move  with  equal  velocity  ? 

ANS. — 2  to  1,  because  the  larger  block  is  supported 
by  two  -parts  of  the  cord,  and  has  in  consequence, 
double  the  force  exerted  upon  it  of  the  smaller  block. 


THE   SCREW. 


181 


P 


rj  nff.  19 


Screw. — In  the  screw, 
like  all  other  simple  ma- 
chines the  power  x  space 
moved  through  =  weight 
X  space  moved  through. 
Ex. — Length  of  lever 
20  ins.,  pitch  of  screw  ± 
inch,  weight  500  Ibs.,  re. 
quired  the  power  P  at 
the  end  of  the  lever  ? 
ANS.  Px 20x2x3.1416 

=  500  X  T 
125.664P  =  250 

P  =  1.989  Ibs. 


20 


Tig.  20 


The  screw  is  simply  a 
revolving  inclined  plane, 
the  power  being  applied 
parallel  to  the  base  of 
the  plane,  which  is  repre- 
sented by  the  circumfer- 
ence described  by  P,  and 
the  height  of  the  plane 
by  the  pitch  of  the  screw. 

Fig.  20  is  a  compound 
screw.  The  upper  screw 
AA  is  fitted  to  the  thread 
in  the  nut  B  which  re- 
mains fixed.  The  cylin- 
der AA  being  hollow  has 
another  screw  C,  of  a  finer 
thread,  fitting  into  it. 
The  nut  D  is  fixed,  al- 
lowing C  to  slide  up  and 
down  within  it,  without 


182 


THE    WEDGE. 


turning.  By  this  arrangement  it  will  be  seen,  that 
when  the  screw  A  A  is  turned  once  round,  the  distance 
ascended  by  the  weight  will  not  be  equal  to  the  pitch 
of  AA,  but  the  difference  between  the  pitch  of  AA 
and  C. 

EXAMPLE. — Pitch  of  AA  £  inch,  of  C  TV  inch, 
weight  16000  Ibs.,' required  the  power  P,  applied  20 
inches  from  the  centre  ? 


ANS.— P  x  20  X  2  X  3.1416  =  16000  X  TV  -  TT 
125.664P  -  1000 

P  =  7.957  Ibs. 

In  order  to  multiply  the  power  the  same  number 
of  times  with  a  single  screw,  the  pitch  would  have  to 
be  T]g-  inch,  which  would  render  the  thread  too  weak 
to  withstand  a  heavy  pressure. 

Wedge.— LetWW, 

*  2  *™  fig.  21,  be  two  weights 

of  1000  Ibs.  each,  rest- 
ing upon  a  horizontal 
plane,  required  the 
power  to  be  applied 
at  P,  to  the  wedge, 
having  the  dimensions 
shown  in  the  figure  to 
"^  to  separate  them  ? 

P  X  20  =  1000  X  2 
20P  =  2000 
P  =  100  Ibs. 

Because,  when  the  power  P  has  descended  to  the 
point  A,  the  weights  have  been  separated  2  inches 
while  the  power  has  travelled  20  inches,  the  length 
of  the  wedge. 


w 


Tig".  21. 


FOECE,  TEMPERATURE,  AND  VOLUME  OF  STEAM.      183 


Table  of  the  Elastic  Force,  Temperature,  and  Volume  of  Steam,  from  a 
Temperature  of  80°  to  387.3°,  and  from  a  Pressure  of  one  to  410  Inches 
of  Mercury. 


Elastic  force  In 

Tempera- 
ture. 

Volume. 

Elastic  force  in 

Tempera- 
ture. 

Volume. 

inches  of 
mercury. 

>ounds  per 
sq.  inch. 

inches  of 
mercury. 

pounds  per 
sq.  inch. 

1 

.49 

80 

41031 

53.04     • 

26 

243.3 

1007 

1.17 

.573 

85 

35393 

55.08 

27 

245.5 

973 

1.36 

.666 

90 

30425 

57.12 

28 

247.6 

941 

1.58 

.774 

95 

26686 

59.16 

29 

249.6 

911 

1.86 

.911 

100 

22873 

61.2 

30 

251.6 

883 

2.04 

1 

103 

20958 

63.24 

31 

253.6 

857 

2.18 

1.068 

105 

19693 

65.28 

32 

255.5 

833 

2.53 

1.24 

110 

16667 

67.32 

33 

257.3 

810 

2.92 

1.431 

115 

14942 

69.36 

34 

259.1 

788 

3.33 

1.632 

120 

13215 

71.4 

35 

260.9 

767 

3.79 

1.857 

125 

11723 

73.44 

36 

262.6 

748 

4.34 

2.129 

130 

10328 

75.48 

37 

264.3 

729 

5 

2.45 

135 

9036 

77.52 

38 

265.9 

712 

5.74 

2.813 

140 

7938 

79.56 

39 

267.5 

695 

6.53 

3.1 

145 

7040 

81.6 

40 

269.1 

679 

7.42 

3.636 

150 

6243 

83.64 

41 

270.6 

664 

8.4 

4.116 

155 

5559 

85.68 

42 

272.1 

649 

9.46 

4.635 

160 

4976 

87.72 

43 

273.6 

635 

10.68 

5.23 

165 

4443 

89.76 

44 

275 

622 

12.13 

5.94 

170 

3943 

91.8 

45 

276.4 

610 

13.62 

6.67 

175 

3838 

93.84 

46 

277.8 

598 

15.15 

7.42 

180 

3208 

95.88 

47 

279.2 

586 

17 

8.33 

185 

2879 

97.92 

48 

280.5 

573 

19 

9.31 

190 

2595 

99.96 

49 

281.9 

564 

21.22 

10.4 

195 

2342 

102 

50 

283.2 

554 

23.64 

11.58 

200 

2118 

104.04 

51 

284.4 

544 

26.13 

12.7 

205 

1932 

106.08 

62 

285.7 

534 

28.84 

14.13 

210 

1763 

108.12 

53 

286.9 

525 

29.41 

14.41 

211 

1730 

110.16 

54 

288.1 

516 

30 

14.7 

212 

1700 

112.02 

55 

289.3 

508 

30.6 

15 

212.8 

1669 

114.24 

56 

290.5 

500 

31.62 

15.5 

214.5 

1618 

116.28 

57 

291.7 

492 

32.64 

16 

216.3 

1573 

118.32 

58 

292.9 

484 

33.66 

16.5 

218 

1530 

120.36 

59 

294.2 

477 

34.68 

17 

219.6 

1488 

122.4 

60 

295.6 

470 

35.7 

17.5 

221.2 

1440 

124.44 

61 

296.9 

463 

36.72 

18 

222.7 

1411 

126.48 

62 

298.1 

456 

37.74 

18.5 

224.2 

1377 

128.52 

63 

299.2 

449 

38.76 

19 

225.6 

1343 

130.56 

64 

300.3 

443 

33.78 

19.5 

227.1 

1312 

132.62 

65 

301.3 

437 

40.80 

20 

228.5 

1281 

134.64 

66 

302.4 

431 

41.82 

20.5 

229.9 

1253 

136.68 

67 

303.4 

425 

42.84 

21 

231.2 

1225 

138.72 

68 

304.4 

419 

43.86 

21.5 

232.5 

1199 

140.76 

69 

305.4 

414 

44.88 

22 

233.8 

1174 

142.8 

70 

306.4 

408 

45.90 

22.5 

235.1 

1150 

144.84 

71 

307.4 

403 

46.92 

23 

236.3 

1127      i  146.88 

72 

308.4 

398 

46.94 

23.5 

237.5 

11  05 

148.92 

73 

309.3 

393 

48.96 

24 

23S.7 

1084 

150.96 

74 

310.3 

388 

49.98 

-24.5 

239.0 

I'HH 

15?,.  02 

75 

311.2 

383 

51. 

25               241 

1044      .   15.-..06 

76 

312.2 

379 

184      FORCE,  TEMPERATURE,  AND  VOLUME  OF  STEAM. 


Elastic  force  in 

Tempera- 
ture. 

Volume. 

Elastic  force  in 

Tempera- 
ture. 

Volume. 

inches  of 
mercury. 

pounds  per 
sq.  inch. 

inches  of 
mercury. 

pounds  per 
sq.  in. 

157.1 

77 

813.1 

374 

254.99 

125 

349.1 

240 

159.H 

78 

314 

370 

265.19 

130 

352.1 

233 

161.18 

79 

314.9 

366 

275.39 

135 

355 

224 

163.22 

80 

315.8 

362 

285.59 

140 

357.9 

218 

165.26 

81 

316.7 

358 

295.79 

145 

360.6 

210 

167.3 

82 

317.6 

354 

306 

150 

363.4 

205 

169.34 

83 

318.4 

350 

316.19 

155 

366 

198 

171.38 

84 

319.3 

346 

826.39 

160 

368.7 

193 

173.42 

85 

320.1 

342 

336.59 

165 

371.1 

187 

183.62 

90 

324.3 

325 

346.79 

170 

373.6 

183 

193.82 

95 

328.2 

310 

357 

175 

376 

178 

203.99 

100 

332 

295 

367.2 

180 

378.4 

174 

214.19 

105 

335.8 

282 

377.1 

185 

380.6 

169 

224.39 

110 

339.2 

271 

387.6 

190 

382.9 

166 

234.59 

115 

342.7 

259 

397.8 

195 

384.1 

161 

244.79 

120 

345.8 

251 

408 

200 

387.3 

158 

V 
731 


THE  LIBRARY 
UNIVERSITY  OF  CALIFORNIA 

Santa  Barbara 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW. 


Series  9482 


SOUTHERN  REGIONAL  LIBRARY  FACILITY 


A     000  628  338     6 


